I usually use a die. If I have, say $5 to spend on an auction, then I use a d6 for the other "players" to see if anyone outbid me.
Michael, you must be thinking of a different game. There is no auction in Haspelknecht.
It sounds doable indeed. You would need to design some mechanism to represent the competition on the action discs and on the developments, but basically that's it.
After an enjoyable game last night, I was thinking about some solo mechanics as well. I'm thinking some cards that you flip for each robo-player once during spring and fall which tells them which development they build (columns 1 to 5; next lowest development; one to the left... or right of their lowest development; etc). They would build for free.
During the action selection phase, I would flip cards as well (take the most discs of a color showing; take the most discs closest to the top tile to avoid taking water; take the most brown...or black...or yellow discs, closest to top tile; etc.)
Then just set your score and try to do better. Or there could be some mechanism to determine how high a score is set by opponents but this would take some more interesting and developed system.
I hope to play a game sometime (solo) and I'll see what I can come up with.