<R92> allows for the use of up to two "common household light switches."
So get a three-way light switch (which I believe still qualifies as "common"), which is basically a single pole double throw switch. Then wire it like this:
Code:
-----
| |
| |
+ ----- | switch
- --- | ===========
| | = = motor
| ---=--o = =======
| = o--=---= + =
|---------=--o = = =
| = = = =
| =========== = =
-------------------------= - =
=======
And rig something up to flip the switch to the top position on deployment and flip the switch to the bottom position when the minibot reaches the target.
Of course, you might say that's easier said than done, but if you weren't going to use the NXT brick, you must have had something that could turn the motors on at deployment and turn the motors off when reaching the target? So use the same thing, but with a three-way switch so you get braking on the way down.