I've been told I can't but I'm not sure if that's right...
for instance I can shoot an hour of 720p footage on my old HD JVC GR-HD1.

Mark - the maximum data rate for DV tape recording is 25 Mbs, which is compatible with HDV like that shot with your JVC. The HVX-200 records its HD as DVCPRO-HD at 100 Mbs, and without further compression cannot be recorded to DV tape. There are large DVCPRO tape recorders which cost big bucks and will record DVCPRO-HD, but otherwise you must use P2 cards with the HVX to record HD.

there are options besides P2 cards, e.g. the Firstore FS-100, some would say the CitiDisk HD, and (what I'm holding out for)...the upcoming Cineporter.

And don't forget - you can also record direct into a computer.