I remember catching a FGW HST from Waterloo non-stop to Reading about 20 years ago during a blockade -- pretty sure it went via. Ascot / Wokingham etc. on the SWT route and on to Reading, but happy to be corrected! Fully appreciate that HSTs and IETs are completely differerent beasts, of course.
During the major Reading work HSTs initially went via Salisbury, Basingstoke, Woking, Byfleet Curve and Staines, but after further gauging they went directly into Waterloo, via Woking and Wimbledon. But during the whole project how many days did FGW actually use Waterloo, did it even reach double figures?
IETs have run Reading to Eastleigh works via Basingstoke for a while now, I expect clearing them to Waterloo will be possible, but will it even be necessary?
The Reading station remodelling included 2 brand new island platforms 12-15, (built offline outside the railway in a “high street” environment), one completely rebuilt island platform 10-11, one realigned side platform 7. A reasonably normal train service ran through the worksites for years, with only a few full blocks probably totalling less than a month in total.
The difference with Old Oak is that during the build they won’t need to have any platforms open, and don’t have to deal with temporary access for passengers at all. Why shouldn’t they also be able to build at least the four relief platforms completely offline before they even start any track work?