I direct you to the Wisconsin state constitution. Article IV, Section 7 (Organization of legislature; quorum; compulsory attendance):
Each house shall be the judge of the elections, returns and qualifications of its own members; and a majority of each shall constitute a quorum to do business, but a smaller number may adjourn from day to day, and may compel the attendance of absent members in such manner and under such penalties as each house may provide.
Article VIII, SECTION 8 (Vote on fiscal bills; quorum)
On the passage in either house of the legislature of any law which imposes, continues or renews a tax, or creates a debt or charge, or makes, continues or renews an appropriation of public or trust money, or releases, discharges or commutes a claim or demand of the state, the question shall be taken by yeas and nays, which shall be duly entered on the journal; and three-fifths of all the members elected to such house shall in all such cases be required to constitute a quorum therein.
Which apparently means, as the person who passed this along to the person who passed this along to me notes, that all the Wisconsin Senate has to do is separate out the portion of the budget bill that involves collective bargaining, pass that with a simple majority, and lo! Problem solved, and no more excuses for Wisconsin Senate Democrats to hide like little scared bunnies.