I have an oddball mouthpiece I'm using for a Mirafone 190 CC that might work on your horn. I've tried several different alternatives, and keep coming back to this one. The history on it, to the best of my recollection, is as follows...
Gene Pokorny was rummaging around in Tommy Johnson's mouthpiece collection. He pulled out an old Burt Harrick (sp?) and liked it - shallow, slight undercut, huge throat and backbore. He called it his "CVSM", or "Charlie Vernon Survival Mouthpiece".
Norm Pearson made a few copies when he was out at Marcinkiewicz, and I have one of those (stamped N5CP). I've contacted them regarding this mouthpiece, and they no longer have any record of it.
Kanstul, however, now has a copy in their CNC machine. There are two variants available - one with an N4 rim (for Tommy), another with a C4 rim (for John Van Houten). I think John's using his on his rotary Neptune.
Now, this thing would split the bell of just about any "normal" instrument. On huge rotary horns that need to be "tamed", however, it lights them up pretty well. The shallowness puts some nice zing into the airstream, the undercut gives the cup some additional volume, and the large throat/backbore combination opens up the low register quite nicely. I tried a C4, but it just backed up too much in the basement.
As is the case with any recommendation, "your mileage may vary". However, if you've got $125 to give this oddball a shot:
http://www.kanstul.com
If you're anywhere near Southern California, you're welcome to swing by and try mine out...
...Dave