PP can't export CC to a file.
But then again, I don't know any any online CC standards anyway. I've never seen any online player that offers the ability to turn such things on and off. And as far as I know, there are no standards requiring CC for Internet video, only for broadcast.
As a broadcast engineer, I would suspect, and it would make sense that online video files would also be encoded with EIA-608 and EIA-708 set forth by the Electronic Industries Alliance and monitored by the FCC ( Broadcast Standard ). This makes sense to me as all existing video programs which already comply with the EIA-608 and 708 standards would then, automatically play closed captioning when encoded to a video file for playback.
The FCC has recently required all online video to include Closed Captioning as per the Accessibility Act of 2010 ( FCC: MB Docket No. 11-154) Subpart 6 of this document appears to define that all devices which may playback video through the internet, must be able to support the CEA-708, or closed captioning for digital broadcast television. CEA-708 is a form of embedded data often referred to as metadata which is contained within the MPEG-2 stream.
Not to get too technical about MPEG streams, but in essence, even though your end up with a single file, there are traditionally 3 elementary streams of data contained within. Each one has a PID, or Program Identifier number which identifies the Video Stream, Audio 1 stream and Audio 2 Stream. Additional elementary streams can be added such as a data stream containing Closed Captioning- or whatever like Dolby Digital. The PID allows a Demultiplexer to split the streams appropriately for playback.
I would hope there is a program out there that would embed the scc or mcc file ( however they are produced ) possibly after you export the sequence to a file. One of my favoite and best kept secret long-term video websites out there is videohelp DOT com. there is a plethera of kewl software to do all weird things to video, maybe even add CC!
All this still doesn't help us with Premiere Pro though...