In my opinion bretonian minis and coppelstone fits very well. I do not have TB miniatures but looks awesome. Could not help you in this matter.
I have an incomplete project to make a crusader army for WMM and reuse to bretonian... so I do not think is a folly idea at all.
What is a westeros?
My thoughs of WMM are really positives. I only play with early renaissance lists, like french ordenance, italian condotta etc... in a sort of battles in Italian wars mood, recreating historical battles (pavia, marignano, fornova and bicoca) and the modifications of WMM gives a new aspect that follow the mood of mediaeval in a amazing way. Very recommendable.
I do not advice to slip the rules to the fantasy setting, but use them for medieval battles. The miniatures of brettonian could be used in both types of games