I've done a bit (uploaded to the download section) on cybernetics, but for SM2 rather than SM:P. However, I would use a rule I mention in that combined with a house rule (I think it was a house) that we use when someone has two armour types on. Such as might be gained from using an armour psion when wearing armour as well, or an armoured cyber limb.
The first is the rule of 9s. This is a quick and easy way to divide up body mass/area/volume. Essentially, for total body mass, legs and torso each count as 2/9 of the total and arms and head count as 1/9 each. So when you have a localised effect (like your armoured cyber-limb) that needs to affect something more generalised (the character's defensive capability), you can work out the effect and then divide it appropriatesly by coverage.
The second rule when using multiple ATs. We used to rule that the actual AT was whichever armour was the higher of the two, and the second counted as a DB bonus - we played around with the figure, but ended up with 3*AT as being a reasonable amount. So, a teep normally wearing AT12, bangs up an AT20 arnour psion. He then becomes AT 20 (36) for the duration of the psion.
However, because in your scenario it isn't a full body coverage, I would then apply the rule of 9s. If the example above becomes an armsman wearing AT20, who has a cyber leg armoured to AT12, he would be AT20, and apply a defensive bonus equal to (3*12) * (2/9), or 36 * (2/9), or 72/9 = 8. So his armoured cyber leg gives him an additional DB of 8 overall.
If the situation were reversed (i.e. the cyber leg had a higher AT than the regular armour, I would probably adopt the same principle but make him AT 8 ( 8 ). Or indeed AT 1 ( 8 ) if he is just wearing normal clothes.
However, another matter is if the character is relaxing on a beach in his speedos when someone kicks sand into his face (either metaphorically or actually), so there is actually no other AT to take into account. In that one you can either go with waiting until the critical result to see which body part is affected (which would mean having to resolve the attack against AT1, rolling the crit, and if it affects the cyber area go back and rework the result against AT 12, potentially reducing or removing the actual critical entirely. Which is a pain.) or just give the PC a 2/9 or 18% chance for the attack to use the leg AT rather than the speedo AC. Which could also be a pain but in an entirely different way.
So, for your beach party encounters, encourage your cybered-up PCs to cover up against the sun's UV output. It reduces headaches when they get into trouble later on
