Only water soluble substances can get absorbed intranasally. The same with sublingually. If you aren't going to take them rectally, as you proposed, the best way to do it would be to load up on an enzyme inhibitor such as tagamet (cimetidine) - which you can get OTC, and by taking a lot of fatty stuff with it, such as whole milk, a fatty meal, etc, which will enhance absorption.
To be honest 10mg isn't a lot considering you are coming off narcotics. But take the cimetidine no matter what route and wait at least 45min to an hour and then take it.
Are you looking to feel just the diazepam or do you just want effects regardless, because you asked if there is anything else you can use...
Because you can get doxylamine (the generic unisom in the orange box) and take 50mg with it to potentiate it. Just don't take more as it is an anticholinergic akin to diphenhydramine.
That's really all you can get OTC.
This should provide some comfort.
You can take it sublingually. Some will get absorbed that way if you want to feel some immediate effects but then you just hit a wall, as the majority of it will take quite a while to slide down to your stomach.
If you take it orally crush the pill up and put the powder on your tongue and drink a bunch of milk with it.