I think this is what Summit670 is referring to: (Though you'll need some way to control the air supply)
I'll be darned if a chuck looking exactly like that wasn't in the Harbor Freight ads today.
Another little piece that will work....if you bought one of those little air tool sets similar to what Walmart sells - it normally has a regular air chuck in it and a few various other pieces just for blowing off dust or whatever. One of the little pieces that comes in that set is a little black, rubber cone-shaped tip that screws into one of the little guns. That tip works pretty well for trying to seat a tire on your rim. You just remove the valve stem core and then you can hold the rubber tip against the end of the valve stem and let the air flow! It may not be the greatest but it works if you happen to have one laying around.
I don't know how you would effectively get that to the valve stem on the front tire with both of those discs in the way. It's tough enough with the angled chuck.Well, I always remove the discs so as not to damage them when installing a tire. Doesn't take much to accidentally bend one.