It's a nice idea! However, the syntax 6to105=onlyodd won't allow to choose the numbering style or to assign a fixed value.
Instead, I'm thinking of adding another block to the "range" syntax, like:

6to105even=normal
6to105odd=-

which would produce: 6 - 8 - 10 - and so on.
This seems easier to implement, because by looking only at the range, without considering the value, you can immediately determine if a page is "inside" or "outside" the range*.

(* "range" is an incorrect term in this case, 'cause it won't include all the numbers).

If, say, we don't want to proofread the odd pages, we could either:

• mark them as empty (6to105odd=empty) so that the number is shown but it's not a clickable link
• introduce a new number style called "hidden", which wouldn't even show the number, so we get 6 8 10 12 ecc.

@Tpt, what do you think?

Change 436974 had a related patch set uploaded (by Candalua; owner: Candalua):
[mediawiki/extensions/ProofreadPage@master] Odd/even page numbering through <pagelist/>

https://gerrit.wikimedia.org/r/436974

Change 436974 merged by jenkins-bot:
[mediawiki/extensions/ProofreadPage@master] Odd/even page numbering through <pagelist/>

https://gerrit.wikimedia.org/r/436974