Since I'm going to be doing some towing this summer I've been thinking about a non-adjustable "dump" system tied via a solenoid to the WOT switch. It would dump LPG vapor into the airstream just before the turbo which would do a fairly thorough job of mixing it.
Since I'd only be using LPG while dumping max fuel and under boost (for the most part) I'd be able to make it far les complex.
Take a look at this page:
http://www.mrsharkey.com/lpg.htm