Are there widely accepted rules related to union officers being on faculty senate?
Many faculty senates agree to be governed by Robert's Rules of Order. Buried in it is this clause:

No member should vote on a question in which he has a direct personal
or pecuniary interest not common to other members of the organization.
For example, if a motion proposes that the organization enter into a
contract with a commercial firm of which a member of the organization
is an officer and from which contract he would derive personal
pecuniary profit, the members should abstain from voting on the
motion.  However, no member can be compelled to refrain from voting in
such circumstances.
Citation: http://www.cityethics.org/content/roberts-rules-has-conflict-interest-rule-local-governments-no-conflict-provisions

Note that it says that members cannot be compelled to abstain and so there was no violation per se. So in your case, I think the best thing to do would either to work within the Senate to all for an inquiry possibly leading to an impeachment or vote of no-confidence in the leadership.
If you want to avoid direct confrontation, you could leak this to the administration which would be happy to do the inquiry for you, but this would be seen as highly uncollegial and quickly make you unpopular.
How about asking the Chronicle of Higher Ed to look into this as a third alternative?
Edits: Clarifications and obfuscations.