Plugins like ARC II or Sonarworks work well for room eq fix, but only work if you insert them to the Sonar master bus (or the same for any other DAW).
However, I'm forced to insert the same plugin into any other software that plays audio, so I can get this fix.
But not all software has support for these external plugins, like the media players in general.
I think the simplest way would be to leave the correction plugin OUTSIDE and AFTER any audio software. That is, it would look like the last step before the sound came out of the reference monitors.
In this way, any software will have the same equalization, not just the DAW.
Does anyone know if there is any software that can run these plugins and put them after DAW in a backgroud format?

Not an elegant solution but you could leave ARC on the master bus in Sonar and route things like the Windows Media player to Sonar via. Virtual Audio Cable eg. Voicemeeter or Voicemeeter Banana.

I’ve searched but can’t find a simple standalone VST host for ARC.

I have both ARC2 and Sonarworks set up to pass all of my Windows audio through. 
 
I do it with a combination of Pedalboard2 and VAC. 
 
Pedalboard2 will host your VST's. VAC will route Windows Audio where you want it, i.e. to Pedalboard2. 
 
It took me a little trial and error to get the settings right to make it work, but that's probably just because it takes me a little trial and error to get anything right. Also, make sure you use the 64 bit version of Pedalboard2 if your plugins are 64 bit. You can't mix and match. 
 
Then, after I got sick of having to launch and close Pedalboard2 and VAC and change my audio source in the Windows settings every time I wanted to toggle them on and off, I used NirCmd to write a little batch file to turn everything on and off. I have these two batch files in a folder in my taskbar for easy access. 
 
Works great. Now I can listen to Spotify, YouTube, Netflix or whatever through ARC2 and Sonarworks. Heaven!

I found the best solution here: https://youtu.be/4kD-sBH-eQc
Using Cantabile to host the ARC plugin and Voicemeeter Banana to make the correct audio directions, I can now have all the software using the same sound result.
Within Sonar, I only modified Preferences -> Audio -> Playback and Recording -> Driver Mode for WDM/KS (as opposed to the original ASIO), in addition to setting the output driver for "Voicemeeter vaio 1/2".