The 79th wore most anything. Its amazing to see these guys in a frock coat, trews and kepi. A Sack coat and Trews! I love seeing original images of them in miss matched uniforms like that because....it shows that they were not as federal as some would insist.
Both the Company of Military Historians and noted CW artist Don Troiani have reproduced the "field expedient" look of the 79th in pieces of art, Rachel -- have you seen them? If not, you need to.

But I should point out something here: the 79th were Federal Volunteers -- you seem to be implying that they weren't. Some of their uniforms were not standard federal issue (perhaps that's what you mean), but they were not alone in this -- the various Zouave regiments tended to keep their uniforms throughout the war and not adopted federal-issue kit.

Again, check out the CMH and Troiani's work on the 79th.

Cheers,

Todd