We really need to reconsider the role of 'instead' in this joke. I seems to work for one side of the innuendo but not so well the other.
I think this works a little better:
My colleague can no longer attend next week's Innuendo Seminar, and I was wondering if anyone was available to fill her slot?
(By the way, DO NOT accuse me of using the accursed 'Oxford comma'. The comma above is providing additional separation for the two clauses linked by the 'and' conjunction, and NOT adding a wholly unnecessary comma to a list.)