The more I think about it, it seems like a sort of medieval approach would not incur much power usage over just that of leaving the bluetooth on all the time.

In the Alarmed thread, it was mentioned that alarmd runs all the time with no additional overhead just for having more entries. I am running a script via alarmd that plays sound files of clock chimes every 15 minutes. When I first began using it, I checked to see what the difference was in battery usage, & it was unnoticeable.

It seems like a program which just checked to see if a trusted BT device showed up as in range would consume less power than playing sound files, even if the program were called every 5 minutes.

I suppose a quick & dirty test mule for assessing battery use could be run by creating a command line program that just checks to see if a particular trusted BT device showed up as in range, then entering it as a command execution type in Alarmed with a cron string to run it every 5 minutes, which looks to me like:
*/5 * * * * *

I think it was mentioned that having BT set to Visible consumes more power than if BT is on without being set to Visible, so for such a test period it seems like BT should not be set to visible.