This is still correct as per my survey. Event attendees are effectively customers so I followed guidelines here: https://wiki.openstreetmap.org/wiki/Key:access#Customer_access 
In the parenthesized "hours" field, I put a comment instead that summarizes the sign (this is correct as per the hours syntax).
The resulting tag is access:conditional=customers @ ("pouze během konání akcí v Multifunkční aréně Pardubice, 2 hodiny před začátkem a 2 hodiny po skončení")
See changeset 167021963