There are a couple of formats that XILINX software creates, or, at least, used to create, which are shared between ALTERA and XILINX. I wouldn't want to bet on it, but I'd guess that JAM format can be used with Altera's software and programming hardware.

Another thing, though I'm not sure it's entirely relevant, is that I've noted that some XILINX boards now use the same connector, perhaps not the same pinout, as the ALTERA programming cables, e.g. ByteBlaster. It may be irrelevant, but may be worth a look. If nothing else, it might be possible to put a crossover cable or the like in the path.

Those "flying lead" programming cables I mentioned earlier, have no programmable logic in them. They simply use a '244 or, in one case, nothing at all, meaning just a pass-through of the relevant signals.

RE