Marshal-Subrogat was a rank that Podious Whympre created for himself after the Lamplighter-Marshal was served with a sis edisserum and had to leave Winstermill to respond to it. Whympre already had the brevet rank of Comptroller-Master-General and was effectively the second-in-command at Winstermill when he assumed command of the lamplighters. Despite giving himself the title of marshal, Whympre's background was wholly bureaucratic and he did not have any military experience. He thus viewed the dangerous predicament that the Conduit Vermis had fallen into in theoretical as opposed to practical terms, ignoring the practical concerns of the military personnel under his command.
The Subrogate portion of the rank appears to be derived from subrogate, a variant of surrogate.