To my knowledge, no there is not an alternate AUR for ARM.
Some of the AUR submiters are very accommodating, such as jguer with his yay package.
He already has entries in his PKGBUILD to accommodate ARM use, namely
arch=('i686' 'pentium4' 'x86_64' 'arm' 'armv7h' 'armv6h' 'aarch64')
That pretty much covers everything.
In the case of polybar, the submitter only lists the following as architectures
So I have to download his PKGBUILD and edit it to include aarch64 and armv7h and since this is architecture specific, I have to do a makepkg on two separate machines to get aarch64 AND armv7h versions to put in the EndeavourOS arm repos.
So I then emailed the maintainer and very politely told him I had already tested the PKGBUILDS for both aarch64 and armv7h and would it be possible to include these architectures in his AUR PKGBUILD. The maintainer never even bothered to reply to my E-mail much less make the inclusion.
So the submiters and maintainers for the AUR all have different attitudes about their PKBUILDS. Some are very accommodating and some are the extreme opposite.
What I generally do if I am interested in a package for ARM that exists in the AUR is to check if they support ARM architectures in the PKGBUILD. If not, I download the PKGBUILD and edit it to include ARM, then simply try to do a makepkg. So far, this has worked on everything I have tried except “brscan4”, but this involved a binary provided by Brother which was of course in x86_64. I didn’t find where Brother offered the source code, so I just forgot about it.