Sure, anything is possible with software!
Because clip-on tuners are common and a tuner plugin exists in MOD (via the MOD web UI), a tuner feature isn't at the top of my list of things to work on, but another user has prototyped something which is terminal based. I'll try to dig up that thread.
As someone who has tried to implement a tuner using an arduino, I can attest that the code that listens to a string played with all of the harmonics that make up that signal and reliably distill it to a single frequency is not as simple as it might appear. There are plenty of solutions out there using Zero-Crossings, Peak Detection, Slope Detection, Autocorrelation, etc. My suggestion would be to choose one that someone has already perfected and plumb in the signal input and display.
Regarding the display, you/we could likely write some fairly simple python code likely using the PIL graphics lib (like the rest of pi stomp) to render a meter. Another very active user in this forum has been working on a redux of some of the LCD graphics that could make it even easier to add a "Tuner" page.
