The only thing "plug:" would do that mmdevapi would need is sample type conversion.
Not only sample type, but sample rate to. For example, dmix plugin have predefined samplerate for mixing, so if WINE send other samplerate to it without "plug:" wrapper, in best case you will get error message and no sound.
FWIW, my custom dmix device for 5.1 Surround output shows up in aplay -L with a colon, as it's defined to accept arguments for the output device instead of hardcoding one, and neither plain 'dmix' or 'default' get listed. The only name without a colon is 'null'.
Seems like devices without a colon in names is only devices defined in /etc/asound.conf or in ~/.asoundrc
dmix51:CARD=NVidia,DEV=1 HDA NVidia, ALC888 Digital Direct sample mixing device (5.1 Surround)
All but "dmix51:CARD=NVidia,DEV=0" is completely useless
Do you can play wav mono 11025Hz directly to your dmix51 device? Without "plug:" wrapper? With my soundcard I can't.
So I can enumerate dmix*: devices to, I even can enumerate all devices, but I see almost no sence in this.