No direct personal experience, but Fynebond, Hxtal NYL-1, Epotek 301-2 and Araldite 2020 are all used by ceramic restorers. Hxtal NYL-1 was used to repair the Portland vase so should be good enough for most purposes. I'm not sure if any of them comes in a double syringe though – it's usually better to mix by weight than volume.

Not what was asked but I’d be tempted to try one of the clear light-set glues which might be drawn in via a fine wire or thin hypodermic needle and can be built up in layers.
Perhaps this sort of thing: ebay
My experience with such items was with dental work using way more expensive kits so this isn't a direct endorsement of the product link but such products have been around for a long time now so i see no reason why it shouldn't work. The lining product I used was as thin as water (after etching enamel with phosphoric acid paste) and then adding layers of colour-matched pastes to complete the fillings.

pgk

I use Bison Epoxy for these jobs

Link

Try Smooth cast products – maybe from benam.co.uk –

Kristal 6 or 30 ( epoxy based)

Kristal

Crystal Clear 20x series ( urethane based)

Crystal Clear

Both yellow in direct sun with long exposure, both remain clear for 5 years so far indoors.

The urethane is easier to use.

How will you keep a low viscosity material in the crack? A fast cure would assist, but the requirements are contradictory – all the epoxies/urethanes I find tend to have long(er) cure times for low visc. types – fast cure types also shrink more, and have higher viscosity.