Odd situation where we need to deliver 1080i but are shooting on red where we can only record progressive and this needs to be delivered on DVCPRO HD tape so post really has nothing to do with the conversion process.
If you're in an urban area, why not print to file progressively and have a duplication house do the conversion and print to tape?
Alternately, you could rent a tape deck and print to it via HD-SDI. I'd look into something cheap and basic from AJA or Matrox for SDI Out. I have a Blackmagic card myself but it doesn't seem to do a 1080p to 1080i conversion in-card.