Molecular Gastronomy Questions
As I understand it the reason for the alginate and calcium is that sodium alginate gels on contact with a calcium solution. In other words if you squeeze a drop of an alginate solution into calcium solution the alginate solution immediately gels up into a sphere/ellipsoid. This is because calcium ions cross-link with sodium alginate to form polymers (i.e. gel).
The problem with just using agar agar is that agar agar, while also a thickener, has no such "on contact" properties. You just heat the liquid containing agar agar, it thickens, and you get a sort of jelly. I don't think I've ever seen spherification with agar agar, although someone more experienced with this stuff might chime in.
(Also, perhaps this is verboten on Chowhound but I do think eGullet is better at this type of thing.)
The 'caviar' made with alginate is liquid on the inside, with a skin formed by the alginate reacting with calcium ions. So they 'pop' when eaten.
Items made with agar agar have the same consistency through out. The usual approach is to mix the liquid and agar agar, and let it cool in a mold or sheet form. I don't know if you could for small spheres by dropping the solution into a cool liquid or not. It might work. Or the drops might just settle to the bottom, and solidify there.
So if you have ready access to agar agar, go ahead a play with it. Just don't count on producing the same sort of popping spheres that alginate 'caviar' recipes would make.
As a side note, I do have the 'caviar' ingredients in a molecular gastronomy kit. My son and I played around with it, and had limited success. One of the difficulties, I think, was in getting the correct concentration. The kit came with small measuring spoons and a table of weight to volume conversions. But I really should get a small scale capable of weighing small quantities.
The kit came with agar agar, though that is also readily available in Asian groceries.