Oly makes flat top 45ACP uppers or complete rifles, collapsable stock or A2.

As far as the mag capacity being limited, there are a few folks making mag well blocks to accept different style mags. The main problem is a standard AR lower is not wide enough for most hi-cap 45ACP mags. The exception is a Cav Arms lower is a little larger in the mag well.

This guy makes lowers to accept grease Gun mags
Here's a link to a guy who has one:
http://www.oa2.org/forums/viewtopic.php?t=4871
That is an OLY flat top 45ACP upper.

Here's a link to the guy looking to sell a dedicated lower to use grease gun mags:
http://www.oa2.org/forums/viewtopic.php?t=4871

Something similar is done with Thompson mags but is has a rail to work with the Thompson mags and the reciever is cut to accept the wider mag as well as drums.