Thank you for contacting us.
About service alerts and station entrances: yes, you can create service alerts as entrances are considered as a location (aka a record in stops.txt with location_type=2). Therefore, you can select the entrance in message EntitySelector with the entrance’s stop_id.
About service alerts and elevators: no, you cannot create service alerts as elevators are considered as a pathway (aka a record in pathways.txt with pathway_mode=5). Therefore, an elevator must be identified with a pathway_id and cannot be selected (yet) in message EntitySelector with a stop_id.
The best way for you to do this is to use the GTFS-PathwayUpdates proposal. This extension hasn’t been adopted yet (only GTFS-PathwayRouting) so according to the amendment process at least one consumer and one producer should be implementing the proposed extension before a vote is called.
I would strongly suggest you to take a look at the proposal and see if you are able to produce the data with that data format. If yes, then I can put you in contact with the community so there might be some data consumer agencies that will be interested in collaborating with you in getting this adopted.
Looking forward to hearing from you on this!
Cristhian Hellion| en, sp, fr | Community Manager | MobilityData IO | mobilitydata.org