Should I worry about copper pipe outside attracting lightning strikes?
Any build up of static electricity in the ground will seek to connect with opposite charge in a storm cloud, and will spark through the line of least resistance. Any high metal structure, whether it is copper or not, is a candidate for the line of least electrical resistance. You may be able to protect your copper support by a higher lightning conductor (sharp spikey, high, pointing up, and well earthed, preferably to the local water table). Also bear in mind that the lightning can jump over short distances possibly to your copper support.

I once observed a lightning strike on a house close to where I live in South Australia. The lightning hit a TV aerial, jumped to the metal guttering around the roof, jumped to metal window frames, and made contact with the house's internal wiring. It was like a bomb had gone off. Every electrical appliance that was plugged in was fried. The guttering kind of exploded, and a sour smell of 'charred house' lingered for weeks. 

So yes, you should be concerned!