Are the seeds really fresh? I don't have experience with Z piperitum specifically, but germination techniques usually span most genera. Scarification in acid would have been best to start with. However, stratification in moist sand for 4 months at approx 40 F will probably be the way to go. Keep an eye on them for early germinators and remove them as they show.

Hi
It is untimely posting But I do anyway.

Sansho seed have to be harvested when soon after seed are mature , let's say around October. If seed left on the tree/shurb too long , seeds are not germinate well.

If you want to saw seed in spring, clean seed and keep seed in mosted sand. you can use plastic pot and your wife's/ girl friend's old pantyhose to separate sand and seeds. moist but not wet sand.

keep plastick post out side under eave of garage or unheated strage place untill late Feburary or early marchand . Saw seed directry in ground. or soon after seed are mature,you can saw seeds directry in ground. seed should not be covered too deep. 1/4 deep is good enought. Mulching soil with pine straw or wheat straw. avoid large chunk of pine bark mulch.

if you can use jify pot. it work better since roots are not disturbed.

General speaking, Sansho can't transpalant once tree is mature. male tree and female tree is needed to produce seeds. Identifying tree male or female , you have to flip trees up side down. ^^
when sansho flower apear late march or early april, then you can tell tree is male or female.

you can air layer on second year branch to propagate in small scale production. also you can graft sansho.

Please let's me to know if you want to know more.
mike y