If you can create a pattern that matches your intended answer, you can use the hack posted in the other thread. For example, for 7g(x)+2 → 7d_{ummy}+2, the basic and explicit patterns would be
p.sum(p.product(p.integer, p.literal(`d_{ummy}`) ), p.integer )
p.sum(p.product(p.integer.satifies(`x=7`), p.literal(`d_{ummy}`) ), p.integer.satifies(`x=2`) )