Just answering my own question, long after I asked it :).
Yes -- MAGMA is installed on some of the NSF ACCESS supercomputers. It's hard to find any kind of list, but you don't really need to -- if you email the support staff for any of the ACCESS machines, they seem very happy to install things like MAGMA on request.
I've been using MAGMA on NCSA Delta for the past year.
Cheers,
tom