An easier workaround is a busybox built with its own unicode handling, but w/
CONFIG_FEATURE_CHECK_UNICODE_IN_ENV enabled, and launched without an UTF-8 locale in LANG/LC_ALL.
Code:
[root@dahlia root]# ./busybox_env ash
[admin@dahlia root]# BLAH="héhé" && echo ${#BLAH} && echo ${BLAH:2} && echo ${BLAH:3}
6
�hé
hé