There's no simple way to "multiplex" multiple boot.txt's into one, mostly because there isnt a simple user input method usable by u-boot (except the game buttons, but these are only (easily) usable from C code, which is the predefined boot menu). (I'm not saying it's impossible, I mean since you can load binary data in and run it, nothing is impossible). You can however:
1. Have autoboot.txt (in the first partition) as the safe/normal case which is ran normally.
2. And have boot.txt in the first partition as the experimental case that is ran manually.
3. And have a boot.txt in every other partition in the first SD card (selectable from the menu), if you want to meddle with many partitions/root filesystems, each of which propably needs its own bootargs anyways.
(And i propably wont need to mention that incase all of above fails it will still boot normally from NAND, and that you can have several SDs to switch amongst...)
Obvioysly it isnt exceptionally hard to run an arbitarily named script if you can get to the u-boot console (eg. with the USB serial option or with the breakout board and the normal serial console), but this isnt really that usable/simple (USB works, but needs a linux PC and knowhow) for general public, so...