Is there a way to do real motion blur for volumes? (not velocity blur)

In theory, yes.

http://jcgt.org/published/0005/01/01/paper-lowres.pdf [jcgt.org]

Built in to Houdini, no.

Just a quick theory, needs some investigation:
- use the new retime node and slow down the sim heavily ie 10x (how many substeps do you need, maybe 3-4 is enough)
- render this much longer sequence but with lower AA and other sampling settings
- still use motion blur for better results
- blend each bunch of substep frames into one to get the original length
This really needs some research, maybe you'll end up with not too much longer rendertimes and similar quality (low sampling but frame blending)… and you can also use Optix denoiser, which can work well with such soft, but noisy image content - however they not train with such motion blurred volumetric effects, so artifacts are more likely to appear…

Interesting paper btw.

Thanks guys, I tried to cache and render my sim in 1200FPS (Rocket fire) and then do the optical flow in nuke. It works for now, but I still wish SideFX can implement this to mantra. They already got a pretty good retime node in H17 to generate the subframe data.