Unfortunately I think the answer is no. The JSOM is great and yes like you say similar to the SP JSOM (same backend processquery, similar syntax etc etc), however the methods all simply map back to the old server side PSI API methods. That is the same for JSOM, CSOM and REST (REST being the unlucky one as not all methods have been mapped).
The problem you ...
It's not technically possible for one resource to have multiple assignments on a single task in Project.
The alternatives are:
Create one assignment and update the time-phased data, ie the
assignment will have 8h per day for each day during those absences,
for all other days 0h must be entered.
Create one task per absence. Not ideal but this is probably ...