You have read my mind, because that would be the next antenna which I'm building. I already made one for 1250Mhz hamradio ATV and the concept works.
Regarding you antenna polarization question, that exactly the reason i want to build one. The most nodes will probably use a vertical antenna, but maybe there are also nodes with a horizontal antenna (tip over node, simple horizontal dipole, helix, etc). This antenna is having both horizontal and vertical polarization (because of the circular polarization) so in respect to a -3dB loss it it can handle both. But knowing that the difference between horizontal and vertical polarization is -20dB (for example horz node, vert gw = -20dB), the large dynamic range of LORA can handle that for nearby nodes to receive.
Based on my experience in hamradio, I would say most nodes will use vertical polarization. In hamradio vertical polarization is used for non directional communication where horizontal communication is mostly used for directional communication. But also there is a practical part in that :
Quote : "Polarization — the orientation with respect to the ground of the antenna and the radio waves from it — is most important on the VHF and UHF bands, where signals usually arrive with their polarization largely intact. If the radio wave and the antenna are oriented differently, the antenna won’t receive the radio wave very effectively.
FM operating is done with vertically polarized antennas because vertical antennas on vehicles are much simpler to construct and install. Vertical antennas are also omnidirectional, meaning that they transmit and receive equally well in all directions. These characteristics are important for mobile operation — the first widespread use of FM. To prevent cross-polarization, the base antennas are vertical. This convention is universal.
A popular and inexpensive vertical antenna is the simple quarter-wave whip, or ground-plane, antenna. Many hams build a short ground-plane antenna as a first antenna project.
Operators chasing long-distance VHF and UHF contacts use beam antennas that are horizontally polarized. Many of the long-distance VHF and UHF propagation mechanisms respond best to horizontally polarized waves. If you have an all-mode radio and want to use it for both FM and SSB/CW/digital operating, you’ll need both vertically and horizontally polarized antennas."
Source : http://www.dummies.com/programming/ham-radio/basics-of-very-high-frequency-and-ultra-high-frequency-antennas-for-ham-radio/
When you're interested, the free PDF of the book "Hamradio for dummies", can be download on this link). Recommended reading for every one who wants to get into wireless communication (I have the paper version on my bookshelf ).