We now know that they can breed with mortals to create scions. Thomas isn't totally mortal but may be mortal enough since the White Court are closest to human of all the vampire courts. When Thomas was held captive he was forced to feed on many women. It's been displayed that when White Court vamps are very hungry it creates a rather powerful sexual field around them rather than directed in a single direction. Also Thomas was able to show he could seduce a spirit possessing someone for a short period so that could mean the attraction goes beyond pure mortals. Also in Bigfoot on Campus A scion was fed upon by a White Court vamp
So with Thomas being tortured to the point of having an insane hunger would that be enough to cause Shagnasty to want to breed with him?
Shagnasty would not have let himself come under Thomas's power, remember, if a White flipped Mab they'd effectively have the Winter Court at their disposal, I see no reason Thomas couldn't entrance a shagnasty either, except they don't have usual reproductive drives, even if they are capable of the act... I find it more likely done to generate fear and pain in the victim, hence Grey exists I think.
Something as powerful as Shagnasty (Or Mab) could still enjoy having sex with a WCV without succumbing to it's power. Jim said it would take all of the WCV's to maybe have a chance against Mab on Earth. Also just as in Big foot on campus, it was shown the WCV could possibly feed on a Bigfoot scion without ever being able to drain all it's power
... So Thomas could probably create intense pleasure for Shag without ever being able to control it.