The Ffado team is writing firewire drivers and gets support from maufacturers like Focusrite, Echo, ESI, Mackie, Edirol and Terratec (according to their website). This can be in the form of equipment and/or documentation. I'm not privy how extensive this is and it probably is not as complete for each manufacturer or for the whole range of gear. Then there was RME that had the best drivers but since they went firewire this stopped.
For the record Havoc, the part about RME is not longer specifically true:) I haven't tested it myself but I do have from people that would know that the FW support is already developed, and may very well be already available in open source form.
What you don't seem to get is that linux users do some market study before they buy a piece of gear. That you don't get requests is because they just don't buy your stuff. You don't get complaints from people that are not your customers. There just isn't a reason to buy not supported Tascam stuff when I have a couple of RME cards that are fully supported in the kernel for the desktop and an Edirol FA-66 for the laptop. You don't provide for me so I don't buy from you. There is enough gear that does support Linux. (PC stuff, my DA-P1 is still going strong thank you)
Since Havoc went back to this in a later post I will give a specific example. I have a MBP for a variety of reasons and run OS X on it for my main DAW at this point. That being said since I bought it I purchased an Apogee Duet. I already own RME and I purchased a Focusrite because of Linux support with open source drivers(I will touch onopen vs closed source in a moment). I purchased all of these knowing exactly what I was doing and doing the market research beforehand as Havoc put it, and most of the Linux users tend to ask on mailing lists, or on IRC(Considering I provide a lot of support for Ardour and this is a VERY common question) they get specific reccomendations in many cases. Heck look here for example...
Which provides some known good suggestions(And you'll notice even older cards by today's standards) as well as direct links to the sites to look for more options.
I also have no problem and it is very likely in the near future I will be purchasing more Echo interfaces(I have already purchased many throughout my lifetime) and they get my vote for their support of Linux and other users as well as their quality even though many of these interfaces are used on Windows or Mac systems(Primarily Mac) for theatrical playback.
It isn't just a factor of when people do contact you, out of the manufacturers I have bought from I have written directly to one or two, even though I have purchased much more and ignored even more than that.
However if it makes you feel better I would have no problem emailing Tascam and asking the same question:)
In as far as open vs closed source, there is a reason beyond simple ideals that open source is preferred. The number of problems from closed source graphics drivers is much larger than many of the open source drivers. Especially when dealing with audio workstations. On top of that it may require a new interface to the kernel(ie. Not ALSA) or going with OSS in order to properly due a closed source driver, which not going with ALSA or FFADO means that in actuality it will likely not get supported by Jack, which also means that you have just excluded 95%+ of your target market on Linux, in other words suicide and any audio manufacturer, be they hardware or software that releases without Jack support tends to get laughed at by anyone doing professional audio on Linux and they tend to shoot themselves in the foot doing so even though it seems the 'easier' way to do things.
(Just got his laptop back from repair, sorry about the delayed response)