@Xaver88 this may or may not be of any useful info.
Over in the Matrice side, there's been a few of us playing with Ronin MX, after it's lifespan faded.
It has made it a challenge to find various accessories, some nice accessories too.
The Alternate Power Source Adapter, that plugs into the battery slot... you are correct, a few searches and it's no where to be found now. Best source, watch the EBay Ronin MX listings, and examine the photos in each listing. I've found a few optional parts in kits... and a few emails with Seller at right price will often get them to pull it from kit for a seperate sale.
I'm not sure of your intended direction... but a Gremsy Gimbal T3 or S1v3 will both work nicely with DJI RC and App controls.
Gremsy T3
I'm also pretty certain, you can use the P-Tap port to supply power to payload. The P-Tap is a the same as a D-Tap port... no difference. You can obtain multiple types of cables with a D-Tap port with needed power receptical on the other end.
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The method I was looking for was the secondary battery adapter with power expansion, to power the Sony A7Riii.
This was in one of the Ronin MX Threads in Matrice side.
Looking for the Secondary Power Expansion Base for the Ronin MX. It normally came with the larger battery pak... I’d like either with or without the battery pak. If you have a RoninMX that no longer needs to power additional camera or options, I’d be interested in the Exp Power base. I’m in...
inspirepilots.com
The Secondary Power distributor shows up now & then on eBay... it showed up new on B&H last year.
Quick search... pretty hard to find currently... I did find it at this site.
<p><span style="font-size: small;">Designed for the Ronin series hand-held gimbals, this Battery Power Distributor has a P-Tap power connector that can supply power to the gimbal and other onboard devices (such as a RED camera). The power connector is eas
www.aviteh.hr
The other useful option was the PT51 Hi-Cap Battery 4350mah, those too are getting harder to find.
Again, shows up on eBay at times too.
The Intelligent Battery for the Ronin series is easy to install and simple to charge with the provid...
www.camera.ie
Not exactly what you're looking for... but a few alternate ideas.