It might be a silly question - but "off the bat" I cant see an obvious answer...
Why is there an expansion line to the top of the SC reservior, when 12 inches previous there is a "suction" from the same reservior.
Couldnt this suction act as a send receive line or is there some sort of check valve in it?
Then you could simply disconnect the small expansion line.

The other thought I had in addition to this is, since the SC water doesnt/shouldnt get to the heady heights of 212F+++ that the Radiator water could....(as noted in BB's posts onthe subject) its requirement for expansion is a lot less (maybe +4% at 211F). 4% in 2L/quarts is 80ml (compared to zero) or just under three ounces. If you were to disconnect the expansion tube (as per above), you could isolate the SC system simply by gluing (you could even do it with a "temporary" glue) a 1 1/2 inch thin walled PVC tube/annulus (whatever largest fits in the cap hole) around the riser from the SC riser pipe. I tried to illustrate.
You'd have to drain and dry around the area to glue of course. This tube would extend to above the high water line in the reservior (as close to the base of the cap as possible), thus effectively segregating the two systems. No need for another tank - the tube is your new tank......
(?)