Welcome!
*See bottom of file for changes made.
To install this mod, copy WORLD_explorableCutscene.POD to root folder with rayne1.exe. (OG BloodRayne rayne.exe release should still work, though I haven't tried.)
Then open PCPOD.ini, add number at the top by 1 (increment) and put WORLD_cutsceneExplore.POD (the filename) above the highest POD. file. Something like this:

8
WORLD_cutsceneExplore.POD
PCART.POD
PCART2.POD
PCMODEL.POD
PCSET.POD
PCSOUND.POD
STARTUP.POD
WORLD.POD

For reference a vanilla Terminal Cut PCPOD.INI looks like this:

7
PCART.POD
PCART2.POD
PCMODEL.POD
PCSET.POD
PCSOUND.POD
STARTUP.POD
WORLD.POD

Then, in the game itself, enable level select by either password:
ONTHELEVEL
or
SPAMRAIDDASTARDLYCAN

then Shift + Enter on New Game and select AR_DE_SUBBAY_EXPLORABLE.MSN.

You can either let the cutscene play in full to explore the area after the cutscene has played or press Esc at anytime to abort the cutscene script and explore it as-is at that moment.

As it is a serparate mission file, this shouldn't interrupt your playthrough whatsoever. (Not that I've tested...)

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*Changes made from vanilla level file 

rename Root name of mission in .msn file so it restarts to itself instead of the cutscene-only version if Rayne dies of 'mysterious causes' and the level needs to be restarted:
// Root name of mission
AR_DE_SUBBAY_EXPLORABLE


in .SCR (script) file:
add setSlowMoAvailability(true) and setComboLevel(5,3), because at this point of the game you would've unlocked all the powers and combos anyway / why not?

add a fadeout and "gameplay cam" to init function to end the fadeout manually 
  fade(0,0,0,0,5)
  setCameraModeNormal()

changed SkipIntro to reset cutscene cam and also made it fade in incase anyone has ideas to skip it during a fadeout for some reason
:skipIntro
  letterbox(false)
  setCameraModeNormal()
  fade(0,0,0,0,1)
  goto ToCastle
  
removed level-ending stuff and made it go to main loop instead
:ToCastle
  goto mainloop

added WORLD/XX/ language files and renamed them so subtitles would display properly, as the mission has been renamed
(them being missing doesn't make the level stop working, I just felt like making it feel seamless :) )

the SCR (plaintext) version is included for convenience (you will still need Turfster's PodTools to do anything with it)

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unrelated stuff

DXVK 32-bit d3d9.dll (the game already applies d3d8to9) works to offset some of that CPU workload to GPU if needed.

You may notice the default "menu Rayne" picture appear when loading.
I didn't make the loading screen image appear properly because at this point I spent way too much time polishing this whole thing. Don't write that as an actual difference on the tcrf page itself! lol...

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Enjoy, and take care of yourselves.