I drive currently an LPG converted (in 2006) Jaguar XK, before conversion I got unleaded economy up to 39mpg (jaguar quote for combined was 24)
I've never had any issues with it in over 10yrs and have covered in excess of 100k miles, all at between 33-50 pence per liter here in the UK, so with unleaded being £1.17ppl on average I can travel for 300 miles on a £30 full tank, so in monetary terms 52mpg (a figure that seems to be the norm for the C-HR hybrid). Plus of course I get that V8 sound track!
I've done a couple of LPG sponsored economy runs, the last one in 2011 and on one of those there was a 2008 Prius which performed brilliantly....so it can be done. The problem is where to site the tank in a C-HR as I don't think it has a wheel well??
Natch you'd need iridium plugs and I'd probably go with a flash-lube system just to be on the safe side to prevent VSR.
I'd personally not do the conversion until after the warranty is up, however If a health issue then it'll be a necessity, however please don't ruin the beautiful lines of the C-HR with a filler cap in the body as IMO even a colour coded one will stand out.