Its a quick'n'silly workaround but with a job-scheduler -- couldnt you just schedule the appropriate "add_device.php"-commands? The myriad of flavours of task-scheduling should allow all kinds of flexibility here, and its easy to test, easy to log, easy to follow up...
Observium already uses Cron-entries for polling and housekeeping - there is a way in there already.
The other suggestions of adding a device as "ignored" seems good, but it strikes me as a limited use, single-purpose tool. I'd probably end up adding devices and then forgetting about them..
Meaning visualizing and/or keeping track of "idle/pending/ignored/temporary down/in maintainance" would also have to be adressed. Keeping track of too many shades of colour on the Overview-page isnt good either.
/Paal R/