I originally posted the above question to uMarketingSuite support as an email, here is the response:

Custom segment parameters; Yes, you can soon. In the next sprint that starts on November 23rd and finished three weeks later we will make sure that you can implement your own segment parameters. In that way the sky is the limit in our opinion and you can implement every parameter that you want. So only wait a few weeks and then this is possible out-of-the-box. This will require a minimal amount of coding to setup the parameter and implement the function that returns whether the parameter is ‘true’ or ‘false’. In the next sprint that is shipped on November 23rd you get a new parameter by the way that already handles “Logged in members” vs “Non logged in members” so you do not have to code that anymore 😉