A variable displacement vane pump is actually a great idea now that I thought about it a little more, if lubrication and sealing are not an issue, since you can have internal compression/expansion with it. I imagine lubrication and sealing are issues in practice, but this is an interesting idea!
It could be a supercharger/energy recovery device with instant response mechanical control all in one.
I found these:
http://hpwizard.com/vane-supercharger.html
https://en.wikipedia.org/wiki/Powerplus_supercharger
I feel like in this day and age, it should be possible to implement this with carbon fiber parts and DLC + state of the art dry lubricant like BAM. The vanes' sealing force against the wall can perhaps be controlled with a pneumatic pressure + spring combo (the way I'm imagining this is you need to counter the centrifugal forces as the speed increases, so you use very lightweight vanes, a helper spring to provide pressure at low speed, and then a vacuum pump fighting the springs to reduce pressure as it speeds up or something).
Since the mechanical design is really simple you should even be able to do it at home, assuming you can get PVD coating services.
All that said, I still prefer the simplicity of external EGR, but if you're already strapping a supercharger onto an engine, this feels like it could be a very fun but also feasible DIY project for the neo-steampunk enthusiast.