After seeing how expensive most lost model alarms were, and how inexpensive PIC microchip programmers were, I set off to find the plans for a do-it-yourself (DIY) microprocessor-controlled lost model alarm. After a bit of googling and trolling the RCGroups forums, I found this 12F675 controlled LMA project.
I procured an Olimex PIC-PG2 serial PIC programmer from phanderson.com via eBay for the whopping sum of $13, and some 12F675 chips (and other assorted bits and pieces) from Mouser. I had some servo-extensions laying around, and they would complete the package. The design is very flexible, and performs a number of other functions, such as glitch-counting, low-voltage alarm, and a “beep on demand” function. You can set the low-voltage setting in the code before you burn the chip, and you can build it for either horns or piezo buzzers. This was my first experience building a microchip controlled anything, so I am fairly happy! All told, I think the whole thing weighs well under 1/2 oz, depending on configuration.
I haven't had a chance to work on these any further, as I've been working on my Allegro-Lite for most of the winter, but I intend on hashing a few out for local club-mates.