This value being missing from the hash means that if it is added to the *.nmconnection file, despite being valid it will be removed if any operation triggers the GUI (i.e. like toggling the link active).
This patch adds it to the initial hash table lookup, and includes a freetext field under the advanced tab to allow it to be entered.
Users who need this field will find details online, and the tooltip text makes it clear which OpenVPN parameter has been modified.
I am using this now to successfully deal with a corporate VPN which has SHA1 certificates still enabled (which is likely to be common for the next few years - it is stage 1 of several wrapped auth methods).