Are you successfully using the sparc64-unknown-linux-gnu target today, compiling your own rustc? That's pretty cool - I would have assumed the port was completely busted.
In order to get out of the box SPARC support we'll need to add builders to our CI. There's some documentation of this here. It should be pretty straightforward. Step one is probably to add 'dist-sparc64-linux' to the Travis matrix and figure out what needs to change from there.
The hardest part will probably be cooking up a Docker container containing a working gcc x86->SPARC cross compiler, but there's lots to crib off of.