I think the concern over safety with putting subs behind the seats was based on the assumption that the firewall would be cut... the idea is to remove the carpet and insulation to gain about another inch of mounting depth, but no, the firewall would not be cut into. If you mount both subs low and behind the seats you should be able to do it without getting in the way of the access panel.
I'm only 5'7" and my SO is 5'2", so I've got plenty of space behind the seats. I'm looking at doing this with some shallow mount 10" subs. I've looked at the Pioneer units and I'm not terribly impressed. I've got 600 Watts available for each sub and it's hard to find a shallow mount sub that can handle that. I'm leaning towards the Power Acoustik sub as this is probably the best fit for my power demands:
http://www.poweracoustik.com/pa2006/product_AUDIO_wof_STW.htm
Angman, I'd be interested in your enclosure (probably carpeted though rather than leather wrapped) if it could fit these subs.