Oh, I'm sure there are ways Wink - I'm sure recalling at the time that adding the dye to diesel wouldn't work either...
Quite apart from the kit you need to get the stuff into your tank! We have a kit to do this at the gliding club as the winches have Chevy 502 V8s powered by LPG. The filling kit consists of a compound about the size of my back garden with two huge LPG tanks, piping, transfer pumps, isolators, fire extinguishers, etc. I'm sure it could be done on a slightly smaller scale, but it wouldn't exactly be covert.
.. and someone would get suspicious that your central heating appears to be slightly inefficient if you were running a V6 Omega on it too! All it would take is an extra tier on tax on heating LPG at a little above a reasonable level for a central heating system to use. No need for dyes.
I must admit I considered LPG when I bought my last car 7 years ago and, with hindsight, had I converted it I would have been quids in, but I didn't believe then that the tax would remain favourable so I didn't invest. No one seems to have confidence to invest in anything that's at the mercy of the treasury at the moment..
Kevin