There is a function called sim->flood_prop that can flood fill a property so I think this is what you need.
Here's the text from the wiki:
sim->flood_prop(int x, int y, size_t propoffset, void * propvalue, StructProperty::PropertyType proptype);
What it does: Sets a property via flood-fill. (Like PROP)
int x - Particle's x coordinate
int y - Particle's y coordinate
size_t propoffset - This is a value returned by the function offsetof(Particle, <property>).
Replace <property> with what you want, for example tmp or ctype.
void * propvalue - A reference to your property's value.
One needs to define the value beforehand.
For example, when you've defined int temperature = 200;, then you should use &temperature.
StructProperty::PropertyType proptype - What kind of type the value you provided is.
For example, if you had a temperature number (as an integer), then you could use 'StructProperty::Integer'.The types are: 'StructProperty::Integer',
'StructProperty::Float' and 'StructProperty::Uinteger'