Um.. I'm gonna have to respectfully disagree with your statement.
as an old iron monger.. I've used PLENTY of old implements with leaky seals.
your contention that the front input shaft bearing CAN NOT be lubed by the grease is.. well.. LAUGHABLE to say the least.
For 1, ever hear of JD corn head grease? any idea what that was developed for? gear boxes on corn headers? don't figure they burn up using that grease do ya? It's a NLGI 0# grease.. it flows. You can also get NLGI 00# grease that is more or less a pourable lube a lil thicker than molassis on a cold morning. You can also get EP pourable lube, which has a viscosity somewhere in between the nlgi 00# grease, and say.. 300w gear oil.
I't COMMON practice to fill a gearbox with a pourable grease / lube to combat a weeping seal. when you do this you ov course fill so that the front seal and bearing are still getting lube.
You wouldn't pack a gearbox like this with regular gun grease like a #2 or #4 as it will hog out and form a cavity.. however adding some #2 or #4 gun grease to a box that already has oil in it, in order to thicken it up to prevent a weeps seal is fine.
I have a 10' mower. int he owners manual it states to use an 85w140 gear oil OR a EP pourable grease like a nlgi 00# grease.
I have currently have an old servis 6' hog ( think rhino ), that had weepy seals, but was filled with pourable grease. seals stopped weeping, hog has been beating down brush just like it was it's first 30ys of life. lube level is above the low end of the input shaft. have an old magrix seeder with a gearbox doing the same deal... don't look like it's planning on quitting any time soon either.
while for most equipment, seal replacement isn't bad, and can be done with a generic if you know the shaft OD, housing ID, and depth, I have run into a few applications with specialized seals that without the oem parts.. a generic could not be had without machining the input housing to accept the common unit. for a cheap old clapped out implement that runs fine on grease.. it simply may not be worth it to stick 100$ or more of machining plus the seal cost into what may have been a 'free' deal anyway...
soundguy
One thing for sure is that if it needs a seal, it needs to be replaced. If you just pump grease in the gear box, thats ok for the gears. But there are two input shaft bearings and the grease will not get to them and you know what happens when you run bearings dry. $$$ I know, for I had a old bushhog brand bushhog and thats what happen to it. Fourty dollars of bearnings and races. That was several years ago. Thats why they put 90 w oil in the gearboxs.