okay
so basically expansion line isnt really needed (well cant see an explanation for it at this stage.)
see attached schematic

So my idea of simply inserting/sealing a 1+1/2 inch riser in the present expansion tank could fly - although PVC is out - the arrangement needs to handle;
a) temperature to 220F
b) 100mls of expansion capacity (SLK32 - 2L x 5%)
c) not hinder being able to refill main engine reservior space when required - well you can pour it in slower
d) not hinder pressure relief cap closing/operation for main engine "catastrophic" relief.
e) applied sealant cant eventually degrade and come loose and get caught in water galleries

Obviously a CO2 system (dry ice) would have to be open to atmosphere to allow for gases to escape