freezing LPG?
LPG will only freeze at -190°C. Not really likely in GA!
The freezing that is actually happening is atmospheric water vapour. If you deliver the LPG as a liquid and allow vapourisation near (but preferably within) the combustion chamber, then you could get it to work.
As with all engineering problems, there are ways...
BTW, I suspect the reason it's not used is similar to the reasons that it's only marginally accepted in the auto industry - packaging, safety concerns and ignorance. It actually is close to the most perfect fuel for an SI engine.
A