I saw this on the other board too and responded about the registry (often people don't buy gifts til the last minute), but I didn't notice the RSVP thing. Do the invites actually say RSVP, or is it regrets-only?
If it's regrets-only, the text is a fine idea, but I'd do it today (Monday) or tomorrow, not Friday. If it is actually RSVP, though, I'd take it a step further, because you are well within your rights to check with each guest individually. Not RSVPing is a serious violation in my book!
Here's what has worked for me: contact each guest who hasn't RSVPed, saying something like, "Susie Hostess has been so busy with the shower that she's not sure she's kept up with all the RSVPs, and we don't have yours listed - we are so sorry if you replied and we didn't make a note of it! Should we expect to see you this weekend?"
You can do this via phone, email, whatever... You can even enlist close friends or family members to help contact some of the "outlying" guests (think: your mother should call her friends whom she insisted on inviting although you see them only twice a year, your husband should check with his odd co-worker who awkwardly pushed for an invitation over the water cooler, etc). People are really lazy about RSVPing, and often they need a push - which is super annoying when you have to do twice the work for half the guests!