The B3030 cruise is a lever that pushes the hydro pedal and holds it where ever you stop. It takes some effort to push the cruise lever because it is moving the hydro pedal. You can hold the the hydro pedal at the speed you want to go and then push the cruise down until you feel resistance. The cruise lever will now move easily until it hits the hydro pedal. I assume that your B7800 button just locks the pedal at the position you are holding it. I guess, in theory, you could remember the location you had moved the lever to on a B3030 and move it back to that same position after you turned around. I don't think you are missing much by having the button instead of the lever.
An automobile cruise control is a closed system. A computer monitors the speed of the vehicle and adjusts the throttle to hold the speed steady. The tractor is an open system in that you are just setting a speed by locking the hydro pedal, therefore, the transmission ratio. If a large load is placed on the engine, the tractor will slow down and nothing in the system will correct for that. To truly implement a resume you would need a automobile type of cruise control which would mean adding a speed sensor, some type of servo to move the hydro pedal, and a computer to control the thing. Of course this would not work real well because you run the motor at PTO speed for the mower's sake and you control the speed of the tractor with the hydro pedal. If the mower puts a strain on the engine (tall grass, kid's toys, whatever), the tractor would slow down. The cruise control would push the hydro pedal to go faster (similar to shifting into a higher gear), which would put more strain on the motor, maybe eventually to the point of stalling it.
What you want is something that would just remember the last location you locked the hydro pedal at. Kinda like a block of wood just the right thickness to slip under the hydro pedal so that when you push it all the way down till the block stops it you are at mowing speed. If you had such a device, you could turn around, push the pedal untill it stops, and then hit the lock button. It doesn't sound too hard to fabricate something that would do that. You would have to rig up and adjustable pedal stop. I am thinking of something cam shaped that you could control with a lawn mower throttle cable assembly.
