This would have to be something meant for package development rather than making it easier to run custom services on a one-off basis.
Indeed, that's what I intend as the main purpose. Examples for that would be an Icinga Daemon package (similar to the NRPE package), various Backup solutions (SFTP, GIT based, etc.) that would all spring to my mind that we already did with workarounds like custom scripts and filer/shellcmd help but that would be helpful for all so if it would be easier to start the packaging process and get to know/see the hooks where one could link into pfsense's logic (like config hooks etc.) I think we'd have a chance to get quite a few contributions of really useful packages for all :)