[quote=@Ozerath]
The treaty could also only prohibit indiscriminate bombardment: asteroid strikes, strategic nukes, antimatter bombs, etc. Precise tactical strikes of limited scale against specific targets could be permitted.
[/quote]Yeah, let's say there's a maximum allowed firepower which can be deployed against ground targets. And even then provided the attack is aimed at the target in a place unoccupied with civilians, no saturation bombardment or indiscriminate attacks should be allowed.
Albeit this means anyone with space superiority could massacre the enemy without those being able to fight back.
The solution? It's already there since orbital bombardment is generally banned. Theater shields with power enough to shrug off the treaty-limited tactical strikes are allowed. Which means until the ground forces disable such shields or active countermeasures with similar purpose there's no fear of the enemy decimating you from the orbit.
Long story short this would give everyone an obvious target during battles.
These theater screening shields may also offer protection against enemy long range attacks like missiles and artillery.

As for weapons of mass destruction, they are any weapon with city-busting power or such. Their purpose is to cause massive destruction. These weapons are allowed but limited to situations where the intensity of the conflict makes their use reasonable. Also of course a weapon is considered orbital weapon first and WMD second. If it's a spaceborne weapon or anything similar that is banned by the anti orbital strike treaty then it's banned. As such WMDs are limited to things deployed from the ground or from the atmosphere.
And again, harm caused to civilian populations should be avoided. Environmental effects are also limited so no old school nukes leaving irradiated wastelands or poisons that kill the biosphere. Radiation surge caused by nukes is fine, just no fallout from leftover radioactive dust or such. (so you can of course have oldschool radiological weapons if you want, toxins are also fine if they don't harm the environment more than neccessary)

Lastly immensely powerful weapons which are still below WMD category are called Destructors. They can be freely used even in lower intensity conflicts (unlike WMDs) but environmental effects or harming civilians is still something you must consider. Sub-kiloton tactical nukes are Destructors, for example. So is that nifty superlaser you mounted on your giant super tank.
Due to bureaucratic trickery the main cannon of the Bolo is also Destructor while it's effectively a WMD (and certain applications are limited under that law).

[@Sigma]Are the above extension to the Treaty sounds right to you?
I try to make it consistent while also avoiding abuses and allowing players to do more or less everything they want. Players willingly violating the treaty could be also a nice roleplaying plot device, IMO.