This is similar to something that has been on my P1P bucket list. I look forward to seeing how you make out.
But I would also point out that your approach is fraught with peril not to say, about 5X more expensive than using a simple USB hub.
From the AP Board Wiki Page, that connector you’re point to is the DC Input connector for the AP board. Are you suggesting that you’re going to splice into it? I would consider that unwise.
Let’s be candid, all you’re building here is a 24VDC powered USB hub. If you’re going to build a USB hub at that location, then what advantage is there to using that when there is a perfectly suitable circuit-protected USB power port less than 5cm away? I don’t see any advantage to that approach but I do see lots of jeopardy to tapping into input power.
Please educate me if I am missing something.
Instead, why not consider buying an $8 hub and breaking it out of it’s plastic and then printing a bespoke enclosure to fit your needs? It would be a lot safer, faster and cheaper.
Alternate Approach
Summary
NOTE: I have not built this myself as of yet. So do your homework before going down this rabbit hole.
Another possibly more safer approach, which I was considering experimenting with for my own project, might be to use either a buck converter or step down transformer which has built-in circuit protection. This is far safer.
The Meanwell LRS 100-24 Power Supply that BL uses appears to have at least one rail that isn’t be used.
Terminal 6 looks like it is unoccupied. (taken from the Bambu PS wiki)
Therefore it is available for use as an auxiliary power port. Put even if you did have to splice into another circuit, I’d do it from the power lug not from the end of the AP wire, that you suggested above, who’s resistance is unknown. That lug is also circuit-protected so you have added peace of mind.
Uctronics makes a device that I’ve used for other USB projects. I used their competitors products also that I found for $10 for 5 each off of Ali Express.
https://www.amazon.com/dp/B07XXWQ49N/
These can be found all over the Internet. The fact that it also supports a barrel connector for external power supply was a nice-to-have when experimenting with a design. In the last application I used this, I was building an Arduino li-ion battery analyzer. This device allowed me to keep the circuit isolated from the device while I was building it which made current draw measurements a lot easier. The benefit behind measure current draw is that it will aid in understanding if you are pulling too much power out of the 5VDC.
Suggested tools for this project
Summary
Also, do you have a USB power meter in your tool kit? I found these to be way too useful for diagnosing USB power issues and a necessity when verifying a design or just diagnosing suspicious USB behavior. Note, I am not an EE and these are so easy to use, you don’t need to be an EE.
I’ve got six different variants but this UMC25C below is one I’ve had for over five years and is sort of the grand daddy of them all. If you’re doing any kind of USB power design or diagnostics, this tool is indispensable. They come in a Bluetooth version so you can chart power draw on a laptop or Smartphone. At $15-$35, they are too cheap to not have more than one.
Do an Internet search under UMC25 or UMC24, you will find dozens of these by different makers. Also, YouTube search on these because there’s likely better alternatives today. Rui Deng I believe was the first one to introduce this but all that’s behind this is a the same MPU you might find in an Arduino. So they are real cheap.
I purchased this one last year for $70 and still have a little buyers remorse. It supports PPS which is what I needed and it has built-in power analytics, is made of metal and has a very nice graphing display. But truth be told, at 3x - 4x the price of a UMC 25, it was more fluffer than nutter. 
