P_o_M,

Have you ever seen a pastry chef's bag that she uses for decorating cakes? I actually bought an industrial version of that which was made for using mortar and pointing joints. If you can find or make something like that and use a tip with a 1/8" hole then you may be able to mix neat portland to the consistency of cake icing and just act like you are decorating a cake.

In fact, I have made chocolate castings of small leaves, just as we do with concrete, to make dessert decorations.

Jazzbone
Grout bags are very cool for tufa. Cheap and handy.
You can even use cake decorator tips (the same ones that go in pastery bags)into them and get all kinds of cool results.
I love them and add to my collection with every garage sale.
Making a faux birthday or wedding cake from pigment colored grouts is very doable.
It would be hilarious.
The day would come when you could look out in your yard and say "Someones'left a cake out in the rain".:)

P of M
Here is a link to a grout bag.
Home Depot sell them I think.
Most grouting tips will be too big for fine work.
But those Steel ones used for cake decorating will work just fine.
A product like Quikwall should work great in a grout bag.
Or just use your own 'Neat' recipe of Portland and admix.
Don't forget to wash out your bag right after you are done or you will be going to buy a new one.:)

BTW I made my first grout bags out of old rubber gloves and cake decorating tips.
I always wear out the left one so have plenty of rights laying around

Here is a link that might be useful: Grout bag. Every Tufateer should own one!