Not much about the wi-me, but the solution for the power should quite much depend on how you want to do it.
Depending on power usage, i would use a low-drop regulator or use switching power supply. Pros and cons in both. I don't think that switching power supply will be bad for wireless, as they are used in almost every device nowadays, even in the wireless ones. 400mA is still so much current, that you propably want to use switching PSU.
If i had to maximize battery life, I would of course minimize powerlosses in PSU and try to figure out some powersaving modes for the device. Can the radio be turned off / on by software in Wi-Me? And are there any specs about the power usage in these situations (yes, I was lazy, didn't look through the datasheets as I suppose it wasn't told there because of this thread
Just my 2cents, after hard day at work....