[brazen1]

Is it possible to use Kodi (Nexus) VideoPlayer for the Blu-Ray menus and use an External Player like MPC-HC or MPV for the actual video playback?

Certainly.
 

I make BDMV backups (folders, not ISOs) of some of my movies and wanted to use the Kodi VideoPlayer to bring up the menus but then have an external player play the movie or extra features.

Be aware, if you don't 'contain' all the loose folder/file structure in an iso container, when Kodi scrapes the title, it will deposit artwork within your folder/file structure instead of alongside it.  This basically contaminates your RIP.
I don't quite understand your question but if you're asking if a specified external player can launch from a BD menu... the answer is no afaik.  Whatever player is displaying the BD menu is the one that will be playing audio and video.

I think the "current solution" I've always used is pretty seamless, reliable, and effective.  The feature request of having multiple play buttons on a title to manually select a player sounds nifty and they would be a shortcut for what already exists - Context Menu/Play Using/The Player.  Consider folks like myself that use 5 different players.  That would be 5 different buttons for a skin to have to add in the library for a title.  Not gonna' happen let alone the aesthetic nightmare they/it would create imo.

You do understand player selection can be and is automated simply by pressing play on a title?  This is tailored using the playercorefactory.xml.  Every title automatically selects the correct player via the PCF.xml rules created.  If you wish to override the automated selection, you simply manually select a player via the 'Play using' menu. 

For instance:  I just watched a title that required I use an external player with madVR processing.  It had all launched automatically.  Now I'm done and I want to browse the extras in the BD menu.  The external player cannot handle BD menus - main title only.  So, I manually select the Kodi VideoPlayer this time, since it's not the default for this type of movie, and browse the extras in the BD menu because Kodi does handle BD menus... some... not all.  When it doesn't, it defaults to only playing the main movie too.

Been quite a while since I've posted my journey in this thread.  Much has changed.  For instance, I no longer have a need for .bat files.  Fwiw, currently I'm using:

Kodi v22 VideoPlayer - for everything 4k HDR since I passthrough because there is nothing needed to improve 4k video including tone mapping using my decent display.  Also for anything I want to see BD menus for.

MPC-BE with madVR - rendering for most non-4k HDR video since lesser video can be improved using extra processing that Kodi does not provide.

MPC-BE 2 with MPC Video Renderer - when madVR rendering adjustments are not possible due to HDR passthrough and must be done in the player instead.  Saturation + or - for example.  Player adjustments are permanent.  These adjustments can be done on-the-fly via player keymapping but unlike madVR, they permanently retain on-the-fly settings and if you forget to put them back to default at playback end, the next title would be using these changed settings thus the need for a specific player.

MPC-BE BRIGHT with MPC Video Renderer - same as MPC-BE 2 but for a different permanent adjustment such as Brightness + or -.  Very few titles need a Brightness or Saturation adjustment but I have a few that benefit highly from them.

MPC-HC with no intensive post processing so that GPU resources can be better utilized for other GPU tasks such as WindTrack authoring.