My impression about the funding issue is a little different: the government as I understand will continue to provide funding, but at roughly 50-60% of its original total (which is around $120K including salary, benefits, insurance, and educational costs). So doing 4 years of radiology after neurology would deprive a program 2 years of funding. I know of four residents who have done this without much problem as far as I could tell.
Getting an additional year of credit is very difficult given board requirements, unless you plan to spend considerable time afterwards in radiology fellowships (e.g. 2 additional years in INR or diagnostic neuroradiology). But in these cases, you might be able to carve out a 3-year general radiology plan.
Neurologists can attend neuroimaging fellowships and INR fellowships without doing radiology. The former are usually wide open, but getting to actually bill for your own neuroimaging highly depends on where you work, the board of directors at whatever hospitals you work for, competition from other imagers, etc. The latter is now a very well accepted training pathway, but getting the fellowship in the first place is very competitive.
So if you are really interested in being a neuroradiology, it is FAR easier to simply do radiology--you will save yourself at least 2 years of training time. However, if you absolutely need clinical skills and want to be able to manage your patients in a hospital, then be prepared to work.
B