We're used to the fact that bars below or above consonants have an immediate effect on that consonant - that they mark actual gemination (consonant length) or an immediately preceding homorganic nasal to build a cluster. 

We do have two samples, though, in which Tolkien appears to have considered to have these bars mark that the same consonant is to appear again AFTER the vowel that is placed on top (both samples are from Quenya texts in CV spelling).

The first is a subscript bar under lambe with amatixe (in a Númenian spelling) - we would normally expect this to signify LLI, but it is clearly and explicitely meant as LIL - so LI is read regularly, and then another L is read (below that we have a slightly different spelling of the same sentence where the syllable is spelt out with two lambi). You will notice, however, that there is a dot above the bar, and it seems to me that this might mark the difference. Obviously it doesn't mark "no A following", which is its usual reading in Quenya.

The second case is a bar above (in a classical spelling), that is in this particular document used for all long nasals. But in all other cases, where it's read as we are used to, it is directly on the letter and the vowel is placed on top. In this case the vowel comes first and then the bar follows last on top. 

So both do have a certain kind of logic to them, especially the latter, but I'm not sure whether we might try to adopt them...

The latter (which is unfortunately unpublished so that I only know of it from reports and haven't actually seen it) appears to be a very early source (anto is ND, and súle is NT), so this might easily be a remnant of Tolkien's earlier ideas in which Quenya was mostly spelt fully and bars in on any letter could be used to spell final N, so it might be simply obsolete later.

But I'm afraid I'm not sure about the dating of the first example as I do not own a copy of that book. The method seems quite complex though, and even if it's much later I'm still not sure how wise it would be to emulate such a (possibly) one time idea. 

I'm very curious top read your thoughts! 

Reduplication after vowel?