Disclaimer: I have a looser view of canon than some.
Feasibility: Fair
As far as I understand, it this is how many of these vehicles were initially created (from the Rhino chassis), however, the modifications to be made for a vindicator, for example, might be too extensive too allow for modularity. A whirlwind or predator to razorback or even Rhino on the other hand, might be easier. Attachments for fitting these modules may take up extra (fictional) room... or maybe not. (BTW: The silly door on top allows a couple of your passengers to shoot, which can make a huge difference.)
Heresy: Maybe not
While Mars may make a big noise about such things, they don't seem to exert the same sort (not saying less, just different) of influence as the Ecclesiarchy and it is often their own trained "clergy" which make these field modifications, such as the Ares pattern Land Raider et cetera. With no end-result-vehicles being technically outside of the realm of STC patterning, the modularity might only be perceived as a minor modification. (You'll know you've crossed the line, if your Rhinos start morphing into Dreadnoughts. ;) )
My opinion is: There is a vague possibility that someone might notice a slight deviation from STC in your vehicles and less of a chance that they would do anything about it.
(Of course there is this take on it too: If everyone is buying Land Raider Crusaders, the AdMech will lavish it with blessings, while GW gets your buck (and mine.) If everyone is buying magnets to save money on large numbers of vehicles... they may take offense, but might not want scare off business either. This is pure speculation, of course, and clearly not based on canon.)
Hmmm... you (GMT) are saying that the machine spirit wouldn't put up with it? What if the machine spirit were tied into the module? Then it wouldn't know any different. ;)