a guess, hopefully in the right direction

I'd also guess "in loco parentis", but this would have to be turned into a nominal phrase

(I'm battling with a French psychology text as we speak...!)

Noun 1. locum - someone (physician or clergyman) who substitutes temporarily for another member of the same profession
locum tenens
backup man, fill-in, reliever, stand-in, backup, substitute, relief - someone who takes the place of another (as when things get dangerous or difficult); "the star had a stand-in for dangerous scenes"; "we need extra employees for summer fill-ins"

locum
someone who temporarily stands in for someone else, especially in the medical and clerical professions.
Dutch: plaatsvervanging (de)
French: suppléant(e)
German: Stellvertreter (m)
Italian: sostituto
Spanish: suplente

http://www.allwords.com/word-locum.html

Although, as you know, Deborah, the most common translations for "suplencia" are "substitution" or "replacement," in Harper Collins it also means "etapa" or stage, period of time, etc. Moreoever, in VOX Diccionario de uso del español de América y España by McGraw Hill, on page 1807, it states:

"suplencia n. f. 1. Acción de suplir: ... 2. Tiempo que dura esta acción: durante mi suplencia conocí a importantes cardiólogos."

Mike :)