This strategy should penalize GU-pairs harder in short strings than in longer, where they are better tolerated. It also match what I have seen work in lab. Not in all cases, but in many.
I would like a strategy for GU-placement that says:
Give + 2: No GU-pair next to closing basepair, except if the closing basepair is a GC-pair. And the energy in the quad is -2.5. Give + 1 if this is the case.
Give + 2 No GU-pair as closing basepair, unless the basepair next to the closing GU is a GC-pair. And the energy in the quad is -2.5. Give + 1 if this is the case.
Only dispensation is if the next pair is an AU-pair and the energy of the quad is -1.4. Then give + 1.