In my view that is not directly achievable as the '/opt/bootlocal.sh' process (even though it is run asynchronously) will probably be well and truly "gone" by the time the X server comes up.
Therefore I'd like to suggest you ensure that all components that you are interested in are writing (or 'tee'-ing) into a common log file (e.g. '/tmp/my.log') and then create a little script that gets run automatically when the X server starts (e.g. '~/.X.d/show_log.sh') with a contents along the lines of the following:
#!/bin/sh
LOG=/tmp/my.log
[ -s ${LOG} ] && xterm -e sh -c "less ${LOG}"