I’d think of if differently. Why not create a numpad for your own sqeekboard layout? You can create a layout specific for yourself and have numpad as one of the views available (one extra view). In anycase you’d need to create a squeekboard numpad layout (like us_unmpad.yaml) to have something to bring up, if you want it separate. See:
I have seen on rare occasion where I was filling out an online form which the field specifically only required numbers and squeekboard gave a pure number pad layout. It would be reasonable to think that it can be manually shown through some kind of command.
I got curious and, ho, there truly is one already - it just isn’t named numpad. It can be found from the squeekboard repo: squeekboard/data/keyboards/number/us.yaml
But now the question comes back to, how the original command needs to be changed to find that alternative keyboard layout…
Whether the numeric (or any other layout like email) will be used is determined by the input hints the client sends via the text-input Wayland protocol.
To make the same possible via DBus too you’d need to extend squeekboard’s DBus. I’d rather fix the client though.