With last beta http://beta.plantuml.net/plantuml.jar
you can now define "off" day for a ressource.
You can also force a ressource to work on a usually closed day.
For example:
@startgantt
Project starts the 2019/10/01
{Alice} is off the 2019/10/16
' We are lazy now: you can use for/the/at
{Alice} is off at monday
{Alice} is on at 2019/10/21
[T1] on {Alice} lasts 30 days
@endgantt
Thanks again for your suggestion.
Tell us if it's not what you were expecting !