Enable modification of headers - specifically updating or assigning "tags" to a message

As per the discussion in Synchronet Discussion, sometimes updating a tag (or more specifically assigning a tag to a message) fails as a result of insufficient header space (and if I understand correctly, the next block of space is already allocated).

It would be ideal if the header was relocated to accommodate the tag (not sure if that is possible or reasonable), and thus assigning a value to the tag wouldn't be constrained to a pre-defined limit (as per below).

Alternatively, enable the sysop to "reserve" (additional) space for the header (eg: "x" chars), so when a tag is assigned a value, it is guaranteed to have space to store it (upto the size of "the reserved" (x) amount).