I realise that liver glycogen phosphorylase is allosterically regulated by glucose, and is generally in the phosphorylated active form when glucose levels are low, but the textbooks (both Stryer and Voet and Voet) are ambiguous at to whether the liver isozyme is covalently modified by the signalling cascade triggerred by glucagon, or whether this only applies to the muscle isozyme. It is implied that the phosphorylated 'a' form is the "default" state of the enzyme, which will break down glycogen unless signalled otherwise; is this telling me that the enzyme will be active regardless of whether glucagon is present, or will some glucagon be necessary to maintain the enzyme in its phosphorylated state?

Well for starters, muscle tissue is unresponsive to glucagon; therefore, covalent regulation of glycogen phosphorylase does not depend on this hormone. I am hesitant to call either the a form or the b form the default state. They interconvert. Liver glycogen phosphorylase is under the control of glucagon. Try Nelson and Cox's textbook.