Hey @robmarkcole,
Good news; I got pyarrow to install on 64 bit Ubuntu 20.10. It took a lot of trial and error (and the majority of the weekend), but these steps work for me on a clean install.
# Download the package repo keyring
$ wget https://apache.bintray.com/arrow/ubuntu/apache-arrow-archive-keyring-latest-focal.deb
# Install it
$ sudo apt install ./apache-arrow-archive-keyring-latest-focal.deb
# Update apt
$ sudo apt update
# Install apache arrow headers pegged at version 1.0.1-1
$ sudo apt install libarrow-dev=1.0.1-1 libarrow-python-dev=1.0.1-1
$ sudo apt-mark hold libarrow-dev libarrow-python-dev
# Install streamlit
$ ARROW_HOME=/usr PYARROW_CMAKE_OPTIONS="-DARROW_ARMV8_ARCH=armv8-a" pip install streamlit
For some reason, in Ubuntu, I needed to specify ARROW_HOME=/usr which I didn’t have to do in Raspberry Pi OS 64 Bit.
Another thing I noticed was that the latest arrow libraries are now at 2.0.0, whereas the version of pyarrow in pypi is still 1.0.1, so when installing the headers, I pinned them to match the version in pypi. This is a necessary step, as upgrading those packages to 2.0.0 will break your streamlit installation. 
I really hope that apache arrow starts publishing wheels for aarch64 the same way numpy does. This will save a lot of people these headaches.