Anyone know any specific rules covering the carrying of "inert matter" or "organic matter" and penalties for Essence and Channeling spell-users who sport it?
In the
core RMC rules, in RMC Spell Law, on page 25, there is a list of "Casting Requirements". If the character does not meet those requirements, he cannot cast spells.
According to this list, an Essence User must have less than 20 lbs of organic material and less then 5 lbs of metal on his person (he must also have 1 hand free, and not be wearing armor or a helmet of any type).
For Channeling Users, the amount of organic material doesn't matter, but he must have less than 10 lbs. of metal on his person (and have one hand free, no metal armor and no metal helm).
If the caster does not meet those criteria, he cannot cast spells, period.
Now having said that, there are a number of options, found on pages 34-36 of RMC Spell Law, known as Extraordinary Spell Casting that, if used, allow casters to exceed those core requirements and still cast spells. Option 10.8 on page 36 covers the modifiers to be applied for equipment carried.
And as always, it is the GM who determines which options should be used.