This is not difficult to do. Suppose my desk phone is 103, my pc softphone is 203 and my mobile client is 303. If any extension is in use, I want the BLF for 103 to light. In the file extensions_override_freepbx.conf I would have the following lines:
[ext-local]
exten=> 103,hint,SIP/103&Custom:DND103&SIP/203&SIP/303
Now the hint 103 will reflect the status of any of 103, 203, 303. Editing the override file is less than ideal, you must remember the changes you made in case any future FreePBX updates conflict with your code.