をば is basically the particles を + は combined together. It works like を but places extra emphasis on the object (in theory; in practice this "extra emphasis" might be diluted so that it basically just signals formal style).
So this sort of をば works like the を in "ご協力を！" (as a complete utterance) -- there is an action implied, but the actual verb is left unsaid. In the case of "ご協力を", if you were to add a verb it would be "お願いします" or similar, but in the case of this 前ふりをば it's the speaker announcing what they're about to do, so maybe させていただきます would be closer. (I'm not sure what specific verb would be most usual; in any case, the whole point of this construction is to avoid a specific verb.)
So イベントの前ふりをば basically means "Here's the setup for/information about(?) the event", except said using a certain pattern which originates in formal discourse but is now used as a sort of self-lampooning mock-formality. You can find lots of examples of this sort of thing by searching for ご検討をば, 情報をば etc. on Google.