I've created a mod that changes the start date in the American Career to allow players to see and hear the December 7th and earlier stuff in both the stock game and my radio mods. Someone on the UBI SH4 forum asked if it was compatible with RSRD. Having never used it before, I downloaded it and checked, and my mod will at the least remove the Narwhal as a boat selection.
Since you use December 6th as the earliest start date, which means stuff from the 7th, 8th, or before still can't be used, I would like your permission to edit your CareerStart.upc by adding in an earlier date, and include that in a new version of my mod that will let users choose which one they need. I may also include RFB and TMO versions if needed and I get permission from them.
If the answer is no, that's okay. I didn't want to just do it without asking first in case you didn't want anyone mucking around with your hard work.