A Mastodon API client (tested with Mastodon v2.3.3; untested with Pleroma but likely to work) for Pocket PCs 5.0 or newer. (It could work with 2003, but the TLS library situation is too bad for me to justify targetting it.)
Speaking of TLS, your server needs to present a TLS protocol version compatible with your Windows CE version. Windows Mobile 5.0 supports TLSv1.0, but probably not SNI. 6.x should be much better in this regard.
In debug builds, certificate name validation will be disabled, which helps connecting when the certificate store is out of date, but no SNI means you have no virtual hosts, effectively. Keep this in mind. If your instance admin is unwilling to change settings for the sake of this (I wouldn't blame them) then you have other options like a proxy or setting up your own instance.
Smartphone SDK is possible, but needs a new UI for certain, probably library rework to handle the subset of a subset that is a Smartphone Compact Framework. Sorry Q/Blackjack owners.
If you're crazy as me, feel free to submit a PR/file issues!
Requires Visual Studio 2008 with Smart Devices support, and
Newtonsoft.Json.Compact, the last version of which was 3.5.
Build the solution and correct paths to things, if needed. That's it.
To install, just sync the CAB file over and install. You will need .NET Compact Framework version 3.5; the latest version of which I provide for Windows Mobile 2005 devices here. (Exception messages for .NET can be found here).
You need to acquire a development key; the application doesn't currently support fetching OAuth keys. From Mastodon's web UI, go to settings, then development, then create new application. Call it PocketToot, then once saved, put "Your access token" into the token field of the settings when launching the app, and your instance's domain name in hostname. You can hit "test" to ensure that both the token is valid and you can connect to the server with your device's TLS capabilities.
Once launched, the UI should be fairly self-explanatory. While I try to polish it, there are rough edges and uninmplemented features.
Timelines are manually refreshed.