I think the other problem with applying after you've already done a residency is that the radiology program doesn't get as much money from the government for your slot. I don't know how it works exactly, but I know that a program will get more money for a resident in his/her first residency. If you do categorical Internal Medicine, you'd get 3 years of funding (luckily you're prelim right now so it doesn't count), if you were in General Surgery you'd get 5 years of funding, so if you get a spot after this year you'd be golden. If you do a medicine residency, you'd have used up all your medicare funding (not exactly all but a certain percentage of it) for the residency and the program you'd be applying for would have the decision. A board certified internal medicine grad who we'd take a big hit in the funding department if we accepted or a 4th year med student who we'd get full funding for. You can see why it's so hard for people who've done other residencies to get into rads (easier for the gen surgeon who switches after 1 or two years because they have more years of funding, much harder for the IM person).
I've seen the same thing about the nucs supposed backdoor, I know several who weren't able to get into radiology after a nucs residency for whatever reason.
I'd try like hell this year as it's probably your best shot, but even if you don't get in this year, and you really want rads, keep trying and network like crazy. Spots do open up at times and you might get lucky. I think radiology needs all the dedicated people that they can get.
Good luck.