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