Been using this trick for years. As long as you encoded the .mov with the h.264 codec it should work. There was a time not so long ago that all h.264 .mov files exported from FCP would not import into PremierePro. It simply refused. If you just change the .mov to .mp4 PremierePro would instantly recognize and import those files. This is true of many “smart” monitors as well. We’d send .mov’s to clients for playback on their “smart” monitors in their booth at various trade shows. If the monitor wouldn’t recognize the file we’d instruct them to change the file name to .mp4. Works every time. It’s just a wrapper. The internal codec is what’s important.
The real answer is no. You can’t export to H.264 from your production codec (ProRes, or in your case DVCProHD) without a loss of quality. There will always be *some* loss due to compression.
But what you can do is make good choices along the way and minimize the effect of compression.
So, what I would suggest is use the best encoder out there for H.264: x264. It’s used by all the big players (encoding.com, YouTube, Netflix, Hulu, etc.) not just because open-source and free to use but because it’s the most tuneable and produces the best image per bitrate.
Export your master to ProRes if needed, or drop your DVCProHD into it and choose your preset. It comes with a set of Production presets (standard, max, etc.) which should do you just fine, and will do all the x264 tuning for best quality.
For uploads to web services, like YouTube or Twitter, the service will re-encode your video, so why not give them the best you can before they compress the hell out of it for their delivery?
If you want to take this to the next level, you can build in an Export to Handbrake command from FCPX. I’ve got instructions on how here: