As JohnD said… I’ll second it.
BizTalk does not fit into a standard .net build model. You will have endless problems doing anything that changes the .net version numbers on a continuous basis.
I have seen this requirement before and it’s possible to do using find and replace scripts on the raw text file of the maps and pipelines referencing .net assemblies. I just question the value of this since I think a cleaner approach is to version the MSI’s you use for deployment. (you lose side by side version ability but that’s usually ok since BizTalk doesn’t play will with that either without additional effort)