It would have to start negotiations between the the City, the NCC, Feds and Larco. As a condition to the Château Laurier's approval, the Larco and the feds would have to remove the photography museum and parking ramps in the old tunnel. Cleared for an eventual subway station.
Next, (in like 10 years), we build a STO LRT station in the tunnel/under plaza bridge with entrance/exist stairs at the same spot they were back in the Hull Electric Railway (HER) days (see canopies on the left side of the photo, on Rideau Street). Elevators can go up to the Château Plaza.
https://www.pinterest.com/pin/126100858286597254/
A new pedestrian tunnel can start from the old HER turning loop connecting an expanded tunnel between Union Station and the hotel, creating a subway concourse.
https://www.facebook.com/LostOttawa/phot...8732492/1157670807665475/?type=3&theater
Fro this new concourse, stairs, escalators and en elevator (or funicular) to Rideau Station's west concourse.
The STO station in the old tunnel could probably be done for a fairly decent price (relatively), but the connection to Rideau would be expensive. That part of the project could be done in phases within a decade after the STO line opens.
If VIA ever returned to Union, the project would be even more justifiable.