I'll check my books, but I believe Arsenal directly states that mechanical arms can take cycerware, but that it's up to the GM what ware is available.
My issue with using weapon mounts is that they are vastly different from cyberguns; an anthroform drone with two mechanical arms can have two or even more cyberguns, but as far as I'm aware none of them have a BOD of 6 which is required for 2 (or more) weapon mounts.
EDIT
Aha, found it.
At the gamemaster’s discretion, full arms can be tricked out with accessories just like a full cyberlimb (see pp. 335–337, SR4, and pp. 44–48, Augmentation). The gamemaster has final say over what accessories a mechanical arm can take.
As for cyberlimb accessories I would allow, I'll go through them one-by-one and compare with vehicle upgrades if available.
From the core 4th Edition rulebook:Cyberarm Gyromount: takes up 4 capacity, costs 6k, and adds 3 recoil compensation. Similar to Body Stabilizer, 3k more expensive for R3, but doesn't provide knockdown bonus.
Verdict: I'd allow it.
Cyberarm Slide: similar to a hidden arm slide, has no direct equivalent in vehicle modifications.
Verdict: I'd allow it.
Cyber Holster: similar to the cyberarm slide, no direct equivalent.
Verdict: I'd allow it.
Hydraulic Jacks: Modifies jumping tests and distance, as well as falling damage. Has no direct equivalent.
Verdict: I'd allow it.
Large Smuggling Compartment: takes up 5 capacity and costs 100,000¥ (!); similar to smuggling compartment vehicle modification which takes up 1 slot and costs a much more reasonable 1500¥.
Verdict: I'd allow it, simply because it's stupid expensive and provides no real benefit over a regular vehicle compartment, so if you want to blow your money on it go right ahead...
Cyber-implant weapons: has an equivalent in weapon mounts, but each weapon mount requires a BOD of 3 and concealed mounts are very, very expensive.
Verdict: I'd allow it, on the grounds that a mechanical arm is no different from a cybernetic limb in terms of being able to accommodate cyber implant weapons; weapon mounts to my mind are more custom fit jobs for vehicles which do not have weapon mounts, while a cybernetic arm is similar enough to a mechanical arm that the cyber implant weapons can be installed directly with very little adaptation needed.
Now, from Augmentation for 4th Edition:I'd allow Scanner Systems, Snake Fingers, Telescoping Cyberlimbs, and pretty much every Modular Cyberlimb that can be installed in an arm or leg, as they have no direct equivalents in vehicle mods.