I kind of agree with you, porthos, to an extent, and I see where you’re coming from with the non-biblical element. But I also think that the hymn acts as a gathering song uniting the community (obviously not when reciting alone, but we’re assuming that you would try to choose a song which would be appropriate for communal celebration, hence the load of posts above on the nature of LOTH) at the beginning of worship and therefore the same principles may be applied as would to an opening hymn at Mass.
My bishops’ conference have produced
this document which offers guidelines for choosing music at Mass and they do particularly specify certain psalm settings as an option.
I’m not suggesting this is the only way to approach the hymn in LOTH, clearly a general ‘gathering’ song will often be less appropriate than one which is particularly tailored to the season. In this, I think the last two pages of the document I linked to above, where they offer suggestions for each Sunday to match up with the Mass readings, would be helpful.
All that was just to say, porthos, that I appreciate where you’re coming from and I think you’re right in your preferences, but I don’t think it would actually be wrong to use a suitable psalm as the hymn.