Late to the party, but perhaps the AS7-D-HT was a intermediate model, combining elements of the Atlas with the upcoming Atlas II?
Speaking of appearances, is the Atlas II really supposed to look markedly different than the original Atlas? Sarna.net doesn't have an image gallery for the II so there's not much to look at by way of comparison. There's just the one line drawing, apparently sourced from TRO: 3075. I'm assuming this is where the Darth Vader and wind-up toy references are coming from.
Is it possible that the Atlas II looks more or less like the original, and that the differences visible in the one canonical image we've got of the II is more due to artist interpretation than any significant alterations to the originals' appearance? Other "Royal" mech variants very closely resemble their standard siblings (being indistinguishable from any distance). Isn't an Atlas II essentially a Royal variant of the AS-7D? In any case, the game model for BTCG's Atlas II AS7-D-HT is identical in appearance, as far as I can tell, to the original AS-7D.
Furthermore, the apocryphal, fan made TRO: 3028 lists the Atlas II as a variant on its AS-7D page. There's no mention in the Sarna.net or TRO: 3028 entries that the Atlas II differs in appearance from the original Atlas.
Either way, I think the AS-7D-HT* was part of a limited production run because, IIRC, the cut-scene in Castle Nautilus on Artu showed several Atlases in the background. I find the idea of a cache of several AS-7D-HTs more plausible than the idea of one custom variant among several standard AS-7-DHs. Perhaps testing resulted in the adoption of the LB-X-10 instead and the AC20 test-bed versions were put on ice until they could be brought up to production standard. The Exodus occurred before this could happen and the arsenal on Artu was lost to time. In any case, in the BTCG, there wasn't nearly enough time to modify a standard AS-7-DH by swapping the LB-X-10 for an AC20 between the discovery of the cache and the ensuing battle with the invading Taurians.
IRL, weapons manufacturers almost always produce several identical models of a trial version of a weapon system and the production model (if/when selected) usually differs slightly in appearance. For example, take a look at the trial version of the M-16 and then compare it with the first production version and then later A1-A4 versions. The basic shape is more or less the same, but there are enough differences to be able to identify different versions by sight alone.
Anyway, I'm pretty new to Battetech so I defer to your experience and judgment. I just prefer to imagine the Atlas II as looking pretty much identical to AS-7D from MWO, which, IMHO, is very, very badass.
*I modded the Atlas II Kamea Arano awards the PC at the end of the campaign by replacing the AC20 with a Gauss rifle, the LRM20 for a LRM15, and the ER/pulse lasers with their standard counterparts. This gave me enough free tonnage to install three jump-jets. This created a monster that was just about as mobile as the Highlander 732B (my favorite) and could deliver an Alpha strike that could finish off anything lighter than an assault mech with a single shot aimed at the center torso (and it wasn't unusual to kill assaults with a single Alpha strike too). And, even jumping, it hardly ever got close to overheating. It was a real gem.