You can create a throttle hold by setting the safety value on a channel to the switch to use as a hold. A sticky throttle hold will not release until the throttle output is zero, so you don't turn it off only to have your model take off unexpectedly.

You can use a virtual channel as a switch with that property. The channel uses three mixers. The first one uses the old value of that channel, setting it to 100 or -100 with a min/max curve. The second mixer adds the current throttle output to that value with an offset, so that unless the throttle output is -100 or lower, the resulting switch will be on. The last mixer uses a physical switch to set the channel on if that switch is on, otherwise it leaves it at the last value.

[virtchan1]
name=T Hold
template=complex
[mixer]
src=Virt1
dest=Virt1
usetrim=0
curvetype=min/max
points=0
[mixer]
src=THR
dest=Virt1
offset=-1
usetrim=0
muxtype=add
[mixer]
src=AIL
dest=Virt1
switch=RUD DR0
usetrim=0
curvetype=fixed