Personally, when thinking about implimenting something for Mushclient, back before it got added, I had the thought of using markup to create the "layout" of the design you wanted, then either download that, or have the mud send it. The problem I ran into is that you have to create all custom controls. Why? Because MS doesn't even try to make it easy to use "design mode" for controls, outside their own IDE environments, and that assumes you are using controls that recognize it anyway. Worse, one reason to go with something like Lua, instead of tying the client to something in .NET, or COM, is to, at least in principle, allow someone to port it with a bit less insane recoding. Using Windows "native" system for controls kind of screws that, especially if you actually got design mode to work at all.
I have a vague sense how you could, but its... not documented in anything like a direct fashion. Its more like knowing that you can install front wheel drive, but having *never* seen how the whole system ties into the steering, or being able to find a manual showing how to do it right. Knowing the general idea of how it should isn't the same as actually being able to build it.
I drove myself nuts for a while trying to find "anything" clearer than the vague hint buried in MS' own site (and their non-working link to a demo that supposedly used it), before giving up on the whole idea.
In any case, in general, the best solution is "likely" to take something that already has a huge set of features, and works fast, then try to work out how to correct some of its.. oddities. For example, ages ago you could find "fast" text clients for muds, which supported similar features, including scripting and triggers, but also supported text positioning, which Mushclient doesn't. Its not trivial, but not impossible either, everything in existence that is designed to do rich text *has to* be able to remap a document, if you delete internal tags, change the text, or page up/down. This isn't "much" different than what you need to adjust both the buffer display, and what is on the page. In the case of the old days, ones like Telemate, basically recorded the "final" state of a line, once it was no longer on a page, and treated pages just like page breaks in a document editor.
Point being, you could take an existing client, port it to something less platform dependant, as long as it was designed to start with to be that way, then just add the things that are missing. But, only for the open source ones.
Flash... Seriously, I watch movies via that, and used for that, it seems to *eventually* lag what ever it is in, until it either slows the whole system down, or crashes the browser. It has known problems. An interesting alternative seems to be PHP, with java, which, interestingly, has even been used to make things like EyeOS, which is an entire virtual desktop, which runs in the browser. However, Flash is so annoyingly ubiquitous that it even shows up there, for things where it is "easier". So is, imho, trying to use a skate board and a rope, tied to someone else's car, compared to actually having to drive one, but... lol