You might consider categories for the UFO's
Completing a 90 x 105 Baltimore Album quilt for a show might be more challenging than completing a 45 x 60 Warm Wishes child's quilt.
Although the amount of effort for the maker may be about the same - depending.
I do like the idea of the participants choosing three projects for completion - and they could do show and tell for where the project is now and show progress when progress is made. Maybe someone could take pictures of each project and keep tabs on the progress that way. Maybe even why the project stalled.
How many members are in your Guild? Or how structured it is? Maybe they could have a sub-group?
This post is rather inchoherent - because I'm just typing as thought pop into my head.
I do better when someone is "keeping track" - although any progress should be acknowledged and applauded.
Instead of cash prizes, maybe a Ribbon for Completion could be awarded?
Also - how "complete" does a UFO need to be? For some, if a top is ready to be quilted, that is "complete"
For others, it is complete after it has had a label attached.
If I chose to play, the guidelines I might follow would be:
Choose project.
Show where it is right now.
Explain why project was chosen and why it is not finished yet.
Have photo taken.
When progress has been made, bring it back for more show and tell.
Explain what went well - and what did not go well.
Have photo taken.
Repeat.
When project is done,
Award Ribbon
Take photo.
At end of year, present photos to particpants and have a Guild Album.
Also - does any Guild have albums of the show and tell quilts/projects?