The only way I can think of is:

1) Learn a new command (eg. "Real On") that is a copy of the power on command.
2) Relearn the existing PowerOn command to be a dummy command that doesn't do anything when the TV is off (eg. TV Mute, or some command from some old, other device)
3) In the power settings of your config, add the delay, and then the "Real On" command.

This way the Harmony should issue:
1) PowerOn (which will now do nothing)
2) Delay
3) Real On (To actually turn the TV on)
You may also need to add another delay as the Harmony will no longer be using it's automatic power-on delay setting and will be expecting your TV to accept further commands straight after step (3).

I should also ask why you need to put a delay before turning on a device?