The main question I have...is the "com" device (and other serial
	interface devices) attachable to?  Or is there a better way of doing this?

In order for a device node to allow children to attach
to it, the device must be a bus node.  To do that you
need to add `{ }' to he device node in the files file.

Take a look at the sparc64 config in -current.  It uses:

com*	at ebus?				# `com' driver for `su'
kbd0	at com?					# keyboard
ms0	at com?					# mouse

Eduardo