I am wondering about this in general too, if something is water soluble, will it actually be filtered out by a micron filter? Because when something dissolves, its crystal lattice breaks down into individual ions, atoms or molecules, allowing the substance to form a homogenous solution in the water. So wouldn't the original particle size not matter?
Pregelatinized starch is just starch that has been treated with heat. Unlike non-treated starch, it thickens/gels when you put it in water, even without adding heat. So that's a potential problem.
So I wouldn't really consider it "water soluble" in the technical sense: a gel is not a solution. If I understand correctly, this kind of gel would be a colloid. Injecting it as-is would be a bad idea. I'm not sure whether you could effectively micron filter it, it might clog the filter, especially if there is a lot of it in the pill.
Wikipedia has this to say on the subject of solutions vs colloids:
If a colloid consists of a solid phase dispersed in a liquid, the solid particles will not diffuse through a membrane, whereas with a true solution the dissolved ions or molecules will diffuse through a membrane. Because of the size exclusion, the colloidal particles are unable to pass through the pores of an ultrafiltration membrane with a size smaller than their own dimension. The smaller the size of the pore of the ultrafiltration membrane, the lower the concentration of the dispersed colloidal particules remaining in the ultrafiltred liquid.
To me, that sounds like if something is dissolved in water, it
will pass through a micron filter, and if something is a colloid it will get filtered out (assuming it is larger than the pore size of your filter and does not clog the filter). You might have to use extra water for something that is prone to gelling. Please feel free anyone to correct me if I'm wrong about any of this

. Might be useful for the
Micron Filtering Megathread, since we always seem to go by original particle size and not whether or not the substance is soluble in cold water.
EDIT: Sorry this is so long, but thought I should add, there is a type of starch that is sometimes injected in a medical setting, called Hydroxyethyl Starch, for plasma volume expansion in patients who are in shock. It comes with the following warnings, so I would definitely not inject starch and would definitely use a micron filter.
May alter coagulation and bleeding and clotting times. Adverse Reactions: CNS effects. Headache. Dermatologic. Itching. EENT. Submaxillary and parotid glandular enlargement. GI. Vomiting. Anaphylactoid reactions (eg, periorbital edema, urticaria, intense itching lasting for as long as one year, wheezing; mild temperature elevation); chills; mild influenza-like symptoms; muscle pain; peripheral edema of lower extremities; bradycardia, tachycardia, bronchospasm and non-cardiogenic pulmonary edema. Increased rates of acute renal failure and need for renal replacement therapy. Increased mortality.
Not to totally freak you out - it's probably being injected in much higher quantities, and is not exactly the same kind of starch, but still.