run-in-workspace.sh

#!/bin/bash

# Start a UI app and move its window to a specific Xfce workspace.
#
# Example: Move app xoscope to workspace number 2 (starts at 1).
#
#   $ run-in-workspace 2 xoscope

run_in_workspace() {
    local workspace=$1
    local pid winh
    shift 1

    "$@" &
    pid=$!
    echo "pid: $pid"

    while true; do
        winh="$(wmctrl -l -p | awk "\$3 == $pid" | awk '{print $1}')"
        echo "Winh0: $winh"

        if test -z "$winh"; then
            sleep 0.1
        else
            wmctrl -i -r "$winh" -t "$((workspace - 1))"
            break
        fi
    done
}

run_in_workspace "$@"

Nice idea. To allow for multiple windows, consider:

wmctrl -l -p | awk "\$3 == $pid" | awk '{print $1}' | while read winh; do
    # do stuff with $winh
done