It seems to me that the easiest way to quickly add the SS CX race is to just simply lock out the rear derailleur to the cog you want. Shey Linder has a nice video on how to do this and all it takes on a Shimano rear derailleur is an additional shift cable that you can cut and re-use each week. In the video an additional barrel adjuster is needed because of the derailleur choice--that is not needed for a road Shimano derailleur.
The basic idea is to take the front and rear cables off the bike and center the chain over the cassette cog you want to use. Then re string the new shift cable through the barrel adjuster so that the cable stop hits the end of the adjuster and the free cable end goes through the lock nut. Now you can tighten it up and use the barrel adjuster to get your chain line correct. It takes (in theory) about 5 minutes. If you really want to get it set up specifically, you can dedicate a wheel with your cog and use spacers to mimic the cassette position.
In terms of gearing, I am going to only use the rings on my bike (46/39) and try to get a good combination with those. From what I have read, I think a 39x17 is a good one to experiment with. That is a 2.3:1 ratio and my next best choice would be a 46x21 (2.2:1 ratio). But, the advantage to using the 39 ring is that you can use the 46 as a chain deflector along with the plastic chain keeper on the inside.
Of course it will all depend on the course, but the advantage to using this system is...
1. You have a nice selection of gears and you don't have to spend a lot of money.
2. It is quick.
3. You are using your regular cross bike so you are comfortable on it.
1. You are using a full cassette (weight). But, no more weight than a normal CX bike.
2. You have to re-string your cables each week to get back to geared. But, if you mark them with a sharpie, it should take just a few minutes.