According to the docs, the API currently works such as that it runs the corresponding commands locally: http://www.virtualmin.com/documentation/developer/scripts
However, when it comes to installing web apps, one extremely popular feature is being able to remotely install scripts, possibly on servers that may not even be running virtualmin/webmin. Thus, webmin's clustering or RPC feature would be of little use here.
The need for simple wizard-guided installations of web apps has resulted in a plethora of products and web services being offered, such as e.g.:
All of these support remote-installation of scripts by asking the user to provide FTP/SFTP and mySQL access credentials.
According to the docs, it seems that this is a workflow that could also be supported by webmin/vmin, i.e. by working over SSH/SFTP instead of having the assumption that all operations are local.
Obviously, local operations would still be supported by simply ssh'ing into localhost by default.
In addition, reworking the installer APIs accordingly would also prepare them for supporting migrations over SSH/SFTP.