You are working on v7 or v8? I think you can pass the shop_id in context, to the sale.order.line. And define an onchange function in sale.order.line, on which you can set the product_uom ids to shop_id.uom_ids.

For eg:

in .xml file:

<field name="order_line" context="{'shop_ids': shop_id}"/>

And another way is to use this context value directly in product_uom field domain. Am not sure about this second way, just try. But in onchange you can set the uom_ids I think.

Since you already have the many2many relation betwen the shops and the uoms you can add a domain to the uom field of the sale_order_line:

<field name="product_uom" domain="[('id', 'in', [uom.id for uom in order_id.shop_id.uom_ids])]"/>

Hope that helps.

Regards.

René: Since this question site doesn't allow me to comment other questions/answers than my own, I'm gonna just leave this here.

I thank you for taking the time to answer, but I get an immediate JavaScript error when I try your suggestion. Apparently the token parser hangs at the word 'for'. And even if this worked, wouldn't it require me to save sale.order before the filter would be applied to product_uom? I have an itch that somehow the on_change method is necessary here.