I think granting access to the meritable weaponskills via the unlocking quest wouldn't be too unreasonable, and to leave the merits to augment them. So unlocking the category would give you the ability to use all the weaponskills assuming you had the needed skill, but they would be less impressive than their lower level counterparts without merits in them. Then each merit into one of them would make them become more impressive additional merits currently do.
Example:
Requiescat -
0 Merits - 5-hit Non-elemental damage somewhere around Vorpal Blade (as the previous non-quested weaponskill prior to Sanguine Blade).
1 Merits - 5-hit Non-Elemental damage. Adjust MND Bonus by 14%(13.6%)*.
2 Merits - 5-hit Non-Elemental damage. Adjust MND Bonus by 28%(27.2%)*.
3 Merits - 5-hit Non-Elemental damage. Adjust MND Bonus by 42%(40.8%)*.
4 Merits - 5-hit Non-Elemental damage. Adjust MND Bonus by 56%(54.4%)*.
5 Merits - 5-hit Non-Elemental damage. Adjust MND Bonus by 70%(68.0%)*.
*The number in parenthesis denotes the current capped weaponskill split across 5 merits instead of 4 (because first unlocks as is).
As a sidenote, can we get an adjustment on the stat for this one to something at least somewhat more likely? I can understand a paladin or red mage having solid mind stats, but none of the other five jobs that can use this. I could see Int as a choice simply because it is suppose to be non-elemental (assumed magic) damage, or even Dex modification as it has multiple hits.