Yo !
I know this is an old thread, but it might still be useful for others, even now.
It took me a while to figure this out, even after reading all your posts, but i finally nailed it.
To sum it up, you can group tracks together and then disable the group so that your group only serves as a track selection group, and then you can recall the tracks from that group, by using the actions menu.
Now, to save some time for other Reaper users who would use track grouping for track selection purposes, here's a step-by-step recap of the TRACK SELECTION by TRACK GROUPING method :
1) create your track selection group :
Open the Grouping Matrix (Ctrl + Alt + G) and find an unused group (ex : Group 1)
Close the Grouping Matrix (Ctrl + Alt + G) and go back to your Track Control Panel and select your tracks
Open the grouping dialog window (shift + G)
Choose your unused group
Put at least one flag on it (you can (save and then) load the default flags if needed) : ex: "Record Arm Master"
Untick the "group enabled" box
Rename your group if needed, by clicking the "rename" button : ex : MtkTrackSelection
Close the grouping dialog window by hitting the "close button"
NB : as already suggested earlier in this thread, you can also make your track selection group through the Grouping Matrix directly (instead of going through the Track Control Panel), which is even faster :
Open the Grouping Matrix (Ctrl + Alt + G) and find an unused group (ex : Group 1)
Select your tracks in that group through the matrix (put at least one flag on it)
And disable the group by unticking the group box
Rename your group if needed, by highlighting the column of your group, and then right-clicking to open the Group dialog window for this group, and then by clicking the "rename" button : ex : MtkTrackSelection
Close the grouping dialog window by hitting the "close button"
You can now close the Grouping Matrix (Ctrl + Alt + G)
2) recall your track selection group :
Open the actions menu (Shift + ?)
Filter the search for "select tracks group"
Choose and run the "Group: Select all tracks in group 01" action
Now your tracks from the selection group are selected !
NB : to get a better grasp of how track grouping works in Reaper, you should watch the following video by Kenny Gioia / Reaper Mania :
https://www.youtube.com/watch?v=3tmI88BKnb8