The basic techniques are fairly straightforward - there are algorithms for constructing syllables from English words which are accurate most of the time. I used one in a Prolog song browser I wrote, which give an initial syllable breakdown of the lines for matching to abc notes. Initial because, as Jim points out, the syllables and notes don't always match up as neatly as hymns and their tunes tend to, But it gives you a starting point for manual verification/check/edit. (I just had a quick look at a hymn and even there the word spelled heaven sometimes need to pronounces as one syllable rather than two).

It would be equally easy to count the number of sounding notes on a line as an alternative to the syllables. (If you have abc files for the songs neatly arranges by lines both of these are straightforward).

If I understood the niceties of hymn metrical specification a bit better I might give it a go from my DT abc files.

Mick