I would say that the logical dominating accent for "Meménto verbi tui servo tuo, Dómine, in quo mihi spem dedísti" is the first syllable of "Domine".

The ancient neumes seem to confirm this: up until "Domine", the direction "C" for "celeriter" appears repeatedly, at "Domine" there are regular notes, and then the second phrase is dominated with signs of lengthening. This seems to me to indicate a build up to the dominating accent of the phrase, and a winding-down following it. Disclaimer: That's my armchair, totally amateur semiological analysis, I don't really know what I'm talking about here.

As one who appreciates a certain amount of leeway in interpreting these chants, I'll share my own approach to this here.

When I read the translation as found in the Gregorian Missal (p. 560) and compare the St. Gall neums in the Graduale Triplex (p. 347), it seems to me that there isn't so much a dominant accent as much as a dominating phrase. To illustrate (syllables whose neums imply a hold are in boldface):

Meménto verbi tui servo tuo, Dómine,
in quo mihi spem dedisti.

(Be mindful of your word to your servant, O Lord,
in which you caused me to hope.)

The St. Gall seems to imply a conversational cadence to the first line, without any added emphasis to any syllable. The "Memento" actually appears to be the strongest word on account of its imperative nature, but it comes so soon. The "ne" in "Dómine" is more of a "terminating" hold/lengthening than one of emphasis.

Note all of the holds in the second line, though; where to put the dominating accent is quite unclear. (The hold in "mihi" doesn't imply putting the accent on the wrong syllable, and "sti" in "dedísti" is another terminating hold.) Rather than singling out one syllable, taking the whole phrase "in which you caused me to hope," taken as a unit, seems to strike me as a more proper way to approach this.

My own approach to the ancient neums has been to first approach the chant from the angle of the spoken word and see what rhythm can be gleaned from a spoken rendition of the text; then I look at the neums to see where the composer and I differed in interpretation (if at all). For my schola, I still use the melodies as found in the square notes, though.

wow, that is amazing