I've decided to take a simple approach to journal issues. Instead of marking up like this:

<biblScope type="vol">25</biblScope>
<biblScope type="issue">3</biblScope>

then writing special code to get the desired output, I think it's just simpler all round to mark up like this:

<biblScope type="vol">25, no.3</biblScope>

which gets us what we want without any faffing around.