It be quite difficult to image Beagle on the Martian surface. Even if it landed intact and deployed properly, it's a lot smaller than the best resolution possible with the MGS. At best, it might change the shade of a single pixel (the high resolution imager uses 256 shades of gray instead of color). To get that, you'd have to wait until the MGS passed fairly close to the intended landing site, slew the MGS to put the high resolution sweep over the site (the sweep is only a few kilometers wide), then begin the imaging at just the right time. All of that depends on having a good idea where it landed.

Of course, if Gary is right and it hit the surface doing something like Mach 5, there might be a new crater there. That would be easier to image but still hard to isolate from all the other craters in the area unless that particular spot on the surface had been imaged at high resolution before. Only a tiny percentage of Mars has been imaged at high resolution, but it's possible the intended landing site was.

As to your other question, offhand, I don't know of any such device. What might work would be to use the opposite of stealth technology to increase the apparent radar cross section of landers so they'd be easier to detect on radar. This technique was used on the Quail decoy back in the late 1950s to give a small device the same RCS as a B-52 to confuse enemy radars.