Rather exciting news - Chambers has replied, saying
“Thank you for getting in touch, and apologies for the delay in getting back to you. I’m afraid I can’t speak to why ‘impellant’ is not listed in the Chambers Dictionary or OED. However, we’re hoping to release an updated edition in the next year or two, and I’ve added ‘impellant’ to our running list of words for the editors to consider for inclusion in the next edition.“
I’m on tenterhooks and it would be lovely to think that Azed might be aware.