Ohh... I completely missed where you were using a prebuilt switching PSU. I assumed you had built one using a transformer, rectifier, and cap filter. That's a different animal entirely.
This site ( http://sound.westhost.com/project39.htm ) has some very good design ideas for soft-start circuits. His designs are quite a bit more complicated than what you're proposing. I don't have the chops to offer anything beyond that.
I do still wonder, though.. Why bother? The spec sheet says the PSU already has active inrush limiting. I imagine the quoted figures are for a very short period of time (as a guess, 1-5ms). Any residential wiring should handle that with no trouble. You'll regularly see much worse from common appliances. The only case I can see to limit it further is if you're powering this from a generator or power inverter -- in which case, you're barking up the wrong tree by using this PSU anyway.