+1 to the idea of signing gear but I don't know if I agree with every finished item just automatically having the option.
Seems too easy. 300 guild marks when it will probably NQ is a bit steep...maybe make the signature crystals purchased for a large gil amount... or have every craft have the option to get 'Inscription' ability for 8 or 10k guild marks.
Going the ability purchased via guild marks route means eventually every +1-3 item you see will be signed since it doesn't take all that long to come up w/ the guild points, especially if you've been hoarding them already... kind of takes away from it just a bit where in FFXI you had to purchase the crystals and then hope to get lucky so it was less of a sure thing which in turns made signed HQ gear a bit more rare.