We're installing 5/4 oak t&g flooring/ceiling for our loft. All nailers that I've found have a maximum rating of 3/4 - is it only because the shoe will not fit a deeper tongue or other reason? thanks

yes the fastener will remain higher and not at the T&G..this may cause the next board not to rive tight.
you can remove the shove and see if it will compensate..you may need to make your own..say half the thickness of the original shoe.
but you can try it.

if it don't work..pre drill and rive finish nails..you can also use cut nails..they still make them..but it will be a lot of hand nailing.

a 15 Gauge finish nailer will work.

5/4 oak seems pretty rare..cant say I have ever even seen it.

My great-grandpa and his dad always used cut nails by hand.
Powernail has a palm nailer specially designed for their cleats. Its like $150 and you only can do one nail at a time, but it would work. It is normally used for getting close to a wall.

We may try to fabricate our own shoe? This lumber has been milled from our own farm and has some perfection issues of course. It would be nice to find a nailer that has some automation since everything in the house has been done by hand so far. thanks