Hi there, the easiest way to use custom images in maas is to upload them as a BootResource. You can also use a server assuming it supports the SimpleStreams API. However, if the ISO is just being served statically, a script that downloads the ISO and uploads it to MAAS might be your best option. In order to upload an image to MAAS you can run:
maas $PROFILE boot-resources create name=$NAME architecture=$ARCH filetype=ddraw content=@$PATH_TO_ISO
note the filetype being ddraw for the ISO format.