Well, it gets weirder and weirder
It's a lot of hassle sending something back for a replacement, so I decided to try Panasonic again hoping they would say "oh yes, dodgy batch" or something akin to that.
After speaking to one guy, getting put on hold, getting dropped off the line and going through it all again. I talked to a customer service person for a while, then was eventually put through to an engineer.
The long and the short of it was that he said that a PAL60 signal wasn't supported over RGB, and said this was listed in page 54 of the manual (I don't have it to hand right now). But according to him he said that PAL60 was listed as only being supported via composite or svideo connections.
We had a bit of a heated (though good natured) discussion about the various things that were supported via various interfaces. I told him I knew of people that had exactly the same setup as me (cable and Xbox) and were getting a PAL60 picture out of an AE300, which surprised him as the support for PAL60 is exactly the same. I didn't mention Bererkgoose's setup as I wasn't sure if his setup was on a chipped Xbox or not - which could make a difference to the output.
So I'll go home and just double check the manual, but it would seem like my pj is ok, and I should get the Xbox component cables (which was my plan anyhow). And for all out there that are getting PAL60 through RGB... hang onto your projectors, they're magic ones