Unfortunately, this is not something which will be done.
The 172 bogie is a very different design to a Cl.165 or 168. On the two older fleets, the bearing housing (often called the axlebox) is on the end of the axles. The beam that the Tripcock is suspended from is mounted off the axleboxes. On a Cl.172, the bearing is inboard of the wheel, therefore there is nothing to mount the Tripcock from.
Additionally to this, the infrastructure between Aylesbury and Marylebone (notably down the MET line) has a very limited linespeed. The Cl.165s are limited to 75mph, so are suited to low linespeed infrastructure. We gain much from the Cl.172 (which is a 100mph train) by using them down the western route, as the linespeeds are higher. The Cl.172 fleet are great as 'stoppers' amongst the fast Birmingham trains, as they have high acceleration too.
Finally; we have 8 Cl.172 vehicles, and 89 Cl.165s. There are no plans to obtain any more Cl.172s as a) they are very expensive, and b) manufacturers do not wish to make them any more, as the UK railway is moving towards electrification.