Well, they had asked, albeit sarcastically. Ozymandias glowered, but considered.
"Which is it you want, a bottle of ink men can't read or something that lets you get around the enchantment on Forsythe's?" The former would be easier, but the latter probably wasn't impossible, if he put his mind to it. He'd need the runespore fang for sure, and to get hold of some of the ink itself to see the exact sort of enchantment on it, but both could be accomplished with a trip to Diagon Alley. He could probably work out a way to build a set of glasses that would allow the person wearing them to read invisible ink, including the type of ink used here that was only invisible to certain parties.
If he agreed to do it, though, he had to be sure. It was one thing to fiddle around in his workshop and hit walls and dead ends when no one knew what he was up to; it was another thing to admit failure with an audience waiting to see the results of his work. Safer not to try — or at least, not to let on that he was trying until he was sure of success.
"I don't think it would be in my interest to give you a vehicle for keeping secrets from me," he pointed out. "And anyway, how would I know when it worked? I couldn't test it with myself." Oz got this far, and then realized he didn't have any ready excuse to back himself out of the workaround piece, except that admitting he wasn't sure he could do it. Had he just accidentally talked himself into a worse position than he'd been in ten seconds ago?
MJ is the light of my life <3