The only problem I can see with lashing anything outside the pack is brush - if you're not on trails where brush is encroaching, or heading cross country through foliage choked canyons, shouldn't be any problem with it. I've put collapsed poles in the side pocket and under the compression straps without issue before.

45L is comparatively small. I have a 40L, it doesn't feel small to me. I know a lot of folks consider 40-45 to be an extended day or one night pack, tho.