I guess myself, I would look at firming up the linkage/ creating more drag- if I could. maybe compare to a IPL to see if all were there- such as tension spring etc. I'm not real hip on the notch idea myself. If you had to slow down in quick hurry, would one panic and push into the notch?

I agree with tomplum on NOT using the notched detent idea.
The controls for forward/reverse need to be free enough to permit the brake pedal action to overcome the "hold" or grip of the hydro linkage so you can stop the machine without having to use your hand on the hydro control lever.
Your case sounds similar to my own situation with a White Outdoor Power GT 185 that was given to me in "non-running condition".
After I got it running, about the first thing I noticed was............you had to keep your hand on the "Left fender mounted hydro control" or the control lever would creep toward Neutral.
After posting to this forum for discussion on the problem, it was suggested that a "friction washer" in the hydro control lever linkage might have worn out (or broke up and fell out).
Mine had "broke up and fell out", along with half of the compression spring, so there was nothing to create the friction needed to keep the lever linkage from moving wantonly.
Further, it was suggested by a forum member that perhaps a piece of "old shoe leather" could be cut to make a replacement friction washer or 2.
As it turned out, I ended up using nylon flat washers from a home improvement store for "friction washers".
I also obtained a new compression spring at the same store.
It works like this: The nylon washers touch the upper and lower surfaces of the slotway bracket that the OEM shoulder bolt passes through. Then one or 2 flat washers (number of washers adjust spring pressure to fine tune friction) are used against the lower nylon washer.
The spring exerts pressure to "clamp" the friction washers to the slotway to hold the setting of the range/speed bellcrank but still allow the "scissors" (which work when brakes are applied) to move the bell crank to the neutral position by brake action.
I have no idea how your Roper shift mecanism/linkage are configured, but maybe it is similar enough to the WOP GT 185 for these picures to give you some ideas.

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Great explanation and excellent pictures.

Thanks for the responses! Mownie, great discussion and pics. I'll look into fabbing up something to keep tension and hold position. There is no connection between the brake pedal and trans. The brake pedal applies the brakes and "loosens" the drive belt to the trans-think of a manual trans unit. Again, thinking of a manual trans, depressing the brake/clutch pedal the trans stays as it is which is why I was wondering about the "slotted" arrangement.

From the serial number on the engine this was manufactured in '88 so maybe the early hydros didn't have interconnections to return them to neutral?

Exmar, thanks for describing how your old "Goat Roper" is set up.
Considering that the Roper uses a "clutching idler" to disable the trans upon brake application, you have no worries about using your idea for notching the control slot if you like the simplicity of that arrangement.
I would think you only need to address the forward portion of the slot, so it is not too much of a task to accomplish.
I wish you could post a picture of the dash console so my mind could run wild on some possible options for your modification.