
Hi Claire!

Welcome!

Aw bless poor little licky monster! Yes very likely now its habit. You seem unsure as to the cause, I take yourself and your vet never got to the bottom of it? (no pun intended there after your anal gland comment!)
The 'possible' causes would make a looong list but the those at the top would be Stress,boredom,inactivity, Parasite,injury (dysplasia commonly has licking as a sign) contact Allergy (lesser so food allergy) forgeign object stuck in paw (grass seed a main culprit as they burrow in and migrate!)
Anyhoooo dealing with the 'habit' yes covering is difficult, Have you tried Baby cotton socks and securing them with Micropore tape at the top (across the sock not the dogs fur) to be honest though...I think this just gives the dog something else to stress about and keep nibbling.
I would try to divert his attention, when he starts to lick make a noise fairly sharp to get his attention (but
not to alarm or scold!) Once his attention is away from his licking for 3-4 secs reward him tasty treat (smellier the better,yummy! little bit of liver cake or frankfurter) and just keep working on this making the time longer and longer to treat.
You will have to be consistent to break the habit and it take constant work but will make life soooo much better especially for poor little Schnaunzy! Rome wont be built in a day so be patient think of how hard it is for us to break habits (says the still trying to give up smoker!! ;) ) and if he carries on it could lead to him getting sores (
Lick Granulomas)
A bit more activity will help too, keep him busy, have some games, get him some treat balls to push around, and make sure he's getting out enough.
Im sure others will be along shortly with more tips!
HTH good luck, let us know how your getting on :D