This proposed solution is flawed. It's presented for posterity, as well as a potential source of inspiration for a more-reasonable approach.
Given that encumbrance plays an important role in character management in Cataclysm, as well as that the game stores weight and volume of every object, it seems reasonable that encumbrance of a particular item could be derived from its programmatic definition, partially or completely.
Assuming that encumbrance of an item is the degree to which it makes movement more difficult, it could be assumed that weight is an important contributor to the state of encumbrance. Heavy clothing is naturally more restricting, and vice versa.
Volume of the item is the next important parameter. Puffy and long clothing is more difficult to wear than its thin and short counterparts. The relation seems, at least to a mind's eye, exponential rather than linear, given that the further an external item is from the center of one's gravity, the more energy it would take to move it the same distance.
Given that these two parameters are not the only ones to define an item's effect on the wearer's movements, it may be wise to employ flag- and condition-based encumbrance modifiers:
STRAPPEDitems – holsters, backpacks, sheats; everything that requires straps to remain attached to the body – will be more restrictive
SKINTIGHTitems that are
ATHLETICgain advatange over other
SKINTIGHTitems (i.e. are less restrictive) due to being engineered in ways enabling wearer to perform athletic feats
- items that do not
FITwill provide a higher degree of encumbrance (as is the case currently)
- heavy worn containers – e.g. a duffel bag full of CBMs – will be further more restrictive due to the weight being distributed further away from the center of gravity
See also: encumbrance as a percentage value