I'd like someone with a developer chemistry background to comment on why Dust-Off or similar products might affect the activity of XTOL. It's not obvious to me why the main ingredient would (but I'm not a chemist). OTOH the manufacturer isn't obligated to list every ingredient on the label, so who knows what's in there. (Note-- I've used Dust-Off to preserve XTOL for years and have not noticed any problems with it, but I use it to make pictures, not exposure/density curves.)
I can second the commenter who advised against using CO2. That will tend to acidify the developer (if in a water solution), which is not what we want.
I believe that dry nitrogen has been sold in aerosol-type cans for preserving partially-consumed bottles of wine, whatever those are. Maybe that is still available. Nitrogen is quite inert at room temperature. Though it wasn't a cheap as Dust-Off, as I recall.