tl;dr No additional data of interest, but I explain how I searched, and I can explain the green color.
It seems that there have not been any additional observations of that object, which perhaps isn’t too surprising given how faint it is - 21st magnitude is possible only with big telescopes.
Simbad lists that object under the name of “EQ J103712-274051” but doesn’t list any other names, nor any references besides the IAU Circular already mentioned. That’s a purely coordinate-based designation, with the “EQ” meaning it is based on equatorial coordinates. Simbad says that that designation is internal to Simbad for objects of unknown nature and is to be avoided in the literature.
Looking at ADS, the only citation for the IAU Circular is a 2019 paper by Bonanos et al., which is a catalog of HST variable sources. That seems promising on its face, but digging into the paper it turns out that the sources they flag are different ones, and the citation is in a footnote that says, “This object is unrelated to the transient in NGC 3314 reported in the IAU Circular 7388 by Keel & Frattare (2000).”
I also looked for further observations in Vizier- that can be a good way to find observations of a given source, even if it wasn’t called out specifically in a publication. While that search yields lots of objects within 5”, especially in HST observations, none of them seems to quite match up in coordinates and magnitudes with what Keel & Frattare report in the IAU Circular.
Regarding the green color that drew their attention to the object in the first place, here’s my interpretation of what they are saying there. First, note that HST filters are (usually) named with “F” for filter (as opposed to “G” for grism), a number that gives the filter effective wavelength in nm, and then “N”, “M”, or “W” for “narrow”, “medium”, or “wide”, indicating the width of the filter bandpass. So the F450W filter is similar to Johnson B, for example, and the F555W and F675W filters are green and red, respectively.
From 1999, when the object wasn’t bright, they had blue and red images. From 2000, after it brightened, they had blue, green, and red images. (The 1999 “red” data, F814W, are actually near-infrared, but I’m guessing they used those in the red channel for the Heritage image.) So in combining to make an RGB (color) image, they had two each of red and blue images (one before and one after brightening), but only one green image (after brightening), which they would have needed to weight doubly in the composite to get a reasonable color balance. That’s why that object ended up looking green - they weighted the “bright” green data twice as much as the “bright” red and blue data.