Fred,
I had not thought of using a MUAC chart as you would a weight-for-age growth chart in (e.g.) a growth monitoring program (GMP). we have had considerable success using MUAC in an integrated GMP and CMAM program. In this program we measured MUAC and admit on a single MUAC threshold and watch-listed based on proximity to that threshold. This has proved feasible.
I think the main issue with using MUAC for GMP is that MUAC does not change very much or very rapidly with age after about 12 months of age. There is a lot going on in terms of the composition of the upper arm, tissue mass increases over time with increasing limb-length in children that are not stunting or stunted, but MUAC does not increase very much very rapidly. The fear is that (e.g.) monthly monitoring might not be very sensitive to detect faltering. I think that the best approach would be to do SAM and MAM case-finding using MUAC within GMP programs.
We do see measurable and rapid MUAC response to treatment for SAM. The next thing to work on is a simple monitoring tool. This could be a sort of growth chart as in this early example from the Malawi study:

or a simple algorithm such as:
if current MUAC <= previous MUAC
then [ACTION PROTOCOL]
I think that the best people to lead the design work might be clinical staff with experience in HDU / ICU experience as they will have much experience in monitoring tools and algorithms.
Replacing weight monitoring with MUAC monitoring will allow further decentralisation of CMAM services (e.g. to be delivered by CHWs). I think, with careful design, we could develop a tool that could be safely used by CHWs with limited letter and number skills.