I suppose the difference between this game and XI in terms of implementing signing is that in XI you had to make a decision prior to crafting, and use a special crystal. The same mechanic wouldn't work so well in XIV because you tend to use multiple shards of different types to create end items.
So assuming they made the Inscribe ability as something you could just select from the touch up menu instead of finish... The trouble would be that signed goods would no longer be rare, and you'd probably also need a method of removing the inscriptions (XI had AH listing).
Perhaps Inscribe should consume a guild mark item (as mentioned above the guilds do sell crystals for marks in the same way the XI guilds did, but for the reason listed above, using crystals for signing probably isn't viable).
I liked the Mordant concept for etching in Synergy in XI, although burning an etching into cloth might not be the best idea
Perhaps there could be one-time consumables which Inscribe uses that could vary by item type:
Base Material Metal/Bone:
Mordant Etching Fluid and Guild Stencil Set - 300 Guild Marks - Goldsmithing, Blacksmithing, Armorer, Alchemist Guild
Base Material Cloth/Leather:
Guild Approved Cloth Signature Tag - 300 Guild Marks - Weaving, Tanning Guild
Base Material Wood:
Guild Approved Metal Signature Tag - 300 Guild Marks - Carpentry Guild
Food based consumables:
Guild Approved Rice Paper Signature Tag - 300 Guild Marks - Culinarians Guild
Of course there might be cases when a craft can make an item that the signature item is not available from their guild for the main material used in the synth, but that would just encourage ranking up other crafts (a theme which is present throughout the game).
Extending slightly, each type of material could require a different ability to inscribe, which could be bought from the relevant guild.