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Yes.
Get a version.txt for 6.60, then install mediago on your computer, make sure Pro is active, and connect your PSP. Then activate as normal.
Granted, installing new DLC may not work for some reason (doesn't work for me *shrugs*,) but newly purchased games do so just fine.
Alternatively, you can simply connect via wifi to the PSN, and do so that way. (Makes sure to have the 6.60 version.txt active though.)
I get the same DLC issues this way as well, but again, games are fine.
(I came up with the hard way to activate first, whoops.)