Looked through the manual and the DDK. Didn’t find an answer.
Is it possible to force a custom device to always be routed through the primary controller?
<key>brand</key>
<string>Muxlab</string>
<key>cat</key>
<string>500443 - 8x8 HDMI Matrix Switcher</string>
<key>codes</key>
<dict>
--- snip ---
</dict>
<key>method</key>
<string>lineio</string>
<key>type</key>
<integer>21</integer