I am using the following in a script
wget -q -O - localhost:9981/api/status/subscriptions
to get subscription status information in JSON format.
Look at the bottom of the .c files in https://github.com/tvheadend/tvheadend/tree/master/src/api to find out more api possibilities and try them.
I guess the following found in
https://github.com/tvheadend/tvheadend/blob/master/src/api/api_dvr.c could be interesting for you
{ "dvr/entry/grid_upcoming", ACCESS_RECORDER, api_idnode_grid, api_dvr_entry_grid_upcoming },
{ "dvr/entry/grid_finished", ACCESS_RECORDER, api_idnode_grid, api_dvr_entry_grid_finished },
{ "dvr/entry/grid_failed", ACCESS_RECORDER, api_idnode_grid, api_dvr_entry_grid_failed },
I have not tried all of those, so I am not sure if they will work as the "status/subscriptions"