Having just done this very project a couple months ago (after almost a years worth or reading & research) - I think I can offer some assistance.
You have 3 basic options, the specifics of each will vary by tractor model.
Option 1 - Use existing Rear Remotes / SCV
As you have already stated you simply need a pair of hoses long enough to reach from the rear of the tractor to the front of the loader where the grapple hoses are.
Depending on how you route them and how often you remove your loader, some suggest adding a set of disconnects in these hoses (making them 4 shorter hoses instead of 2 long ones in reality). This will let you leave one pair always on the loader (from the grapple to the disconnect point) and the other pair from the remotes to the disconnect point. Which saves you from having to unhook / clamp / tie the hoses and re-route them every time.
The length(s) of hose in this method don't have to be super precise so using the garden hose method to measure and ordering hoses / couplings online will probably get you the cheapest results.
Option 2 - Diverter Valve
Here you have 2 paths to follow. A Pre-made kit (from some of TBN's members like KennyD or possibly MtnViewRanch) - or- You can build your own.
Building your own may result in some net savings but you have to allow for everything you have to design, engineer, and fabricate along the way that someone else has already done (and refined) if you buy a kit (Mistakes cost money. I know I made a bunch. LOL).
lennyzx11 - already gave a good basic overview of how a diverter works. Typically the diverter is installed in the curl / dump circuit (or you can use the lift/lower circuit - totally your preference) and is mounted close to the bucket on the loader. This results in just a couple short hose runs, fairly easy installation, and a lower price than a true 3rd function.
It works by having a single "momentary" switch installed on your loader control knob. When the switch is Open (i.e. Not Pressed) which ever circuit you tapped works as normal. When the switch is closed (Pressed) the diverter changes flow from the primary function you split to the grapple lid. So for example if you split the Curl / Dump circuit - Moving the stick to dump WITHOUT the button would dump the grapple, WITH the button pushed the grapple stays in place but the lid closes.
The disadvantage to this set-up (in general) is you lose the ability to operate all three functions together.
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Option 3 - True 3rd function
This is the most complex and most expensive of the three options.
Here again you have the same 2 paths of Kit vs Build your own. Likewise I think KennyD & MtnViewRanch can both hook you up here as well. Also depending on who you are getting your grapple from; they may offer a kit as well (one of the most popular ones is from W.R. Long and is used by several vendors & dealerships)
I have a thread over in the Customization forum on my build it yourself 3rd function (which needs some additions as I get some loose ends fixed this week)
http://www.tractorbynet.com/forums/customization/358086-kioti-ck-35-3rd-function.html
The 2 main drawbacks to a true 3rd function are complexity & cost. The biggest advantage to a true 3rd function (again - in general) is the ability to operate all 3 functions together.
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For a 3rd function you will install a solenoid valve instead of a diverter - this valve is usually put in the line that leaves the "power beyond" port on your Front End Loader valve and goes to the rear of your tractor to feed the 3pt hitch (and/or rear remotes depending on the tractor).
From this same valve you will also run 2 lines all the way to the front of the tractor to power the grapple lid.
The way this operates is you have a toggle / rocker style switch (or a 2 button set-up) installed on your loader control knob and pushing the switch one way will open the grapple lid, and pushing the other way will close it.
** - The "fine print"
The reason I added "in general" to both the diverter & 3rd function descriptions is because all of these functions/valves are in series. Fluid flows from one to the next thru the series. This style of flow path gives priority to the functions. The first function in the series uses all the available fluid flow it needs and the leftover (if there is any) then becomes available to the next function down, and so on.
As an example try lifting your loader and dumping the bucket at the same time. Unless you are very light on the stick, the loader will lift and the bucket will barely move or not move at all. This is because the flow capacity of the pumps on Sub-Compact and Compact tractors isn't high enough to supply the full flow demand of the lift & curl functions at the same time.
So when thinking about a diverter or 3rd function, please bear in mind that the comments about using multiple functions at once is "in theory". In actual practice you will most likely only be able to alternate between them.
Knowing this, I still opted for a true 3rd function set-up because the way the controls are situated & operate just felt more natural to me over a diverter set-up (and like lennyzx11 - we don't have rear SCV's yet either).
Hopefully this gives you more material to think about and answers more questions that it creates.
Good luck with your project whatever route you decide to take.
