The REAL problem comes from Doctor Who's assertion that it is virtually impossible to travel between dimensions now that the Time Lords are extinct, since interdimensional travel is so much more common in the Buffyverse and doesn't necessarily lead to universal calamity like Doctor Who claims it will if it happens too often. That's not impossible to overcome, of course...
"Oh, 'dimensions' is such a nonspecific term.....these are different. You can't travel between dimensions on the reality axis, but the metaphysical axis? No problem!"
(How else does the Doctor's cameo in my Firefly mod for EVN make any sense? Well, it doesn't, but it's fun. That seems to be a good enough justification for anything else the Doctor does....)