Glenn,You had mentioned because of his age. Many of the Homeguard operated as or under the provost for a period and then formed Regiments as you stated. Many of the homegaurd were those that for some reason could not muster but, still wanted to do their part.This war had this effect on people.Age being one both young and old. Another type were those who were wounded/disabled and couldn't return service on the campaigns.My GGGrandfather was one of these types with the addition he was in violation of his parole(escaped POW) and could therefore not be exchanged.Without being able to be exchanged he could not participate in anything that directly aided in the war as the Oath of Parole(given by Provost) states.An exception to this Oath falls in Gen.Orders #100 which said he could be used in recruiting which in his case fell under the State Provost Dept..Sp.F.O.#131 Dept. of the Cumberland.
I thought of another book,"Soldiers Life and the Secret Service".Has a chapter on Provost Dept. shows office of Provost Alexandria,VA.1863.Part of The Photographic History of the Civil War,Series, Francis T.Miller ,1957.
"Spies and Spymasters of the Civil War",Markle,1994. Has oaths that were given.Spys reported to the Provost.
If you have access to O.R. Series II Corespondece having to do with POW and of State.
You are quite welcome.Good Luck....