OpenVPN: How to mitigate path MTU issues on a per-client basis

We have dozens of embedded devices installed at customers, all calling home to our OpenVPN service. That works fine in general, but a few of our customers have severe path MTU issues. Our influence on the customers to fix their networks is limited, so we need OpenVPN to deal with it. In a nutshell, my question is:

How can i mitigate the low path MTUs of some clients on a per-client bases, that is without using global settings accommodating the worst case for all clients

Note that our worst case it pretty bad: path MTU 576, drops all fragments, doesn't fragment itself, doesn't honour DF-bit. You see why i'd prefer to not solve this issue globally.

The OpenVPN manpage offers a number of MTU related options, most notably --link-mtu, --tun-mtu, --fragment and --mssfix. But it also says

–link-mtu […] It's best not to set this parameter unless you know what you're doing.

–tun-mtu […] It's best to use the –fragment and/or –mssfix options to deal with MTU sizing issues.

So i started experimenting with --fragment and --mssfix but soon had to realize that at least the former must be set not only client-side, but also server-side. I then looked into server-side per-client config via --client-config-dir but it says

The following options are legal in a client-specific context: –push, –push-reset, –iroute, –ifconfig-push, and –config.

No mention of MTU options!

So here are my more specific questions:

• Why exactly are link-mtu and tun-mtu discouraged? What are the potential problems with these options? Note that i am quite comfortable with low-level IP header munging.
• Which of the options link-mtu tun-mtu fragment mssfix have to be mirrored on the server-side in order to work?
• Which of the options link-mtu tun-mtu fragment mssfix can be used in client-config-dir?
• In case all four options have to be mirrored server-side, and cannot be used inside client-config-dir: Are there any alternatives to combat low path MTU per client?

Notes:

• Parts of my questions have already been asked 5 years ago here, But they haven't really been answered back then, hence i dare to duplicate them.
• The OpenVPN server is currently 2.2.1 on Ubuntu 12.04. We are preparing an upgrade to 2.3.2 on Ubuntu 14.04
• The OpenVPN clients are 2.2.1 on Debian 7.6
• I am happy to determine a customer's path-MTU myself manually
• Currently we cannot test much server-side. But we are building a complete separate test bed, should be ready soon.

I am thankful for any helpful advise.