I have recently been studying Malcolm Young - rhythm guitarist of AC/DC - he sometimes combines up and down strokes with the hammer on of a power chord - A5..D5 E5 - after playing relevant open strings.

What can be helpfull are reggae rhythms - where a chord seems to be struck between drumbeats :
To begin with I used to strum conventionally, but purposely I missed the strings on the downstroke, but struck them on the upstroke.