We get 28 or 35 minute car naps too! Wierd, isn't it?!
We're sort of doing set naps on 2 nap days but it makes J very UT so can't successfully do it for more than one day cos otherwise we get nap and bedtime refusal, short naps, EWs etc. It would have been perfect at 7 months for us though. I don't know if G would quite be ready for it though and it involves having a big nap at 1.30 - 3. We're also up at 6am, if we're lucky! I presume that's no good to you? Will post it if it's useful.
I'll be interested to see what you do cos we'd never get a 9.30 nap (even a short one) and then another nap at 12. If we braved the nap resistance we'd then only get a 35 minute nap anyway and then nothing more for the rest of the day. So I'll be very interested to seek how you get on if you give it a go, and whether it does work eventually once tiredness has built up etc.