Okay here is what I have found:
With Gatekeeper on (note Gatekeeper was introduced in 10.8, and later in 10.7.5) assuming the app was signed properly even if its not by an identified developer you should be able to right click it, open, then when you click open on the prompt you should be asked for admin auth. However you can't get here as the system suggests that "The app’s code does not match the original code that was signed." -- thats kinda a problem but could just be an error in signing it. Haven't really had time yet to figure out whats going on there, might just wait till they fix that part ;p
Okay so lets say you disabled Gatekeeper and you get to the install and it says the certificate is not trusted, well thats because you don't have the Worldwide Developer Relations certificate installed on your system, not surprising (Riot please test your beta on a clean install of the latests OS X w/o developer stuffs installed), go download it here, http://www.apple.com/certificateauthority/ -- click download certificate under worldwide developer relations header....just click the downloaded file and it will add it to your keychain (ONLY do this if you want to trust apps using that cert). Either way after installing the cert you will no longer get an error about Riot's cert being untrusted. That does give some peace of mind 
anyways here is the checksum of the beta I downloaded, shasum -a 256 LoLPBEInstaller.pkg
1a7eaa2351067fee1f3e8c35dba8f3fbd39e34f6b482f9332d 729d81d839c468
Oh and managed to setup a PBE account \o/
Thank you Riot for going forward with a mac client, thats native too \o/ This is really nice as I can finally ditch my ancient winXP VM
After this app leaves Beta, at least think about distributing it via the mac app store.
--- Update ---
Interesting Apple bug, The Gatekeeper KB http://support.apple.com/kb/HT5290 says that the damaged message will appear even if Gatekeeper is disabled at least thats how I read it...but it won't appear if you set Gatekeeper to anywhere.
At the bottom of that KB it suggests spctl and codesign cli tools...
https://developer.apple.com/library/...rocedures.html
spctl --list --label "Developer ID"
5[Developer ID] P0 allow execute
anchor apple generic and certificate 1[field.1.2.840.113635.100.6.2.6] exists and certificate leaf[field.1.2.840.113635.100.6.1.13] exists
6[Developer ID] P0 allow install
anchor apple generic and certificate 1[field.1.2.840.113635.100.6.2.6] exists and (certificate leaf[field.1.2.840.113635.100.6.1.14] or certificate leaf[field.1.2.840.113635.100.6.1.13])
^^ so looks like we can execute and install...not sure why the last part doesn't end with an "exists" though
spctl --assess --type install --verbose LoLPBEInstaller.pkg
LoLPBEInstaller.pkg: CSSMERR_TP_NOT_TRUSTED
^^ okay so here we run into problems, which leads us to, https://developer.apple.com/library/...11201-CH7-SW1/
if you scroll down it starts to talk about that error --- I realize this link is for iOS but I think it might be more helpful in resolving this problem, http://developer.apple.com/library/i...33-CH1-TNTAG19
^^ found that link here, http://stackoverflow.com/questions/1...-trusted-error
anyways hope that helps
UPDATE!! I figured it out, see my last post, http://na.leagueoflegends.com/board/...5#post33839385
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