Thanks for your interest.  Just to make sure we're on the same page, T_A is like a bitcoin transaction with an input of at least 65 units (bitcoins/nooshares) and no explicit outputs.  Essentially, the nooshares used to purchase the computation are destroyed temporarily, but some are later redistributed.  So I'm interpreting your question as "what if no computation has been scheduled for some block?"  Please let me know if I've misunderstood or my answer isn't clear.  The answer is that I haven't decided yet.  The default computation ("curly A" in the paper's notation) would work.  Some publically interesting computation like a protein folding problem or searching for Fermat primes might also make sense.