yeah. So long as it's not completely breaking the setting or playing heavy favoritism to one member of the party, I don't see the slightest problem with it. The point of any game is for people to have fun, and if you need to tweak one of the rules to allow for that.
So, this guy wants to be mostly chrome without being a cyberzombie? That doesn't even seem all that challenging.
If they're prime runners, and he's going street sam, he will probably have Bucket and Buckets of Nuyen. Let him have access to Betaware, but without increasing the Max Availability (Which is at 15 IIRC, though, if you have a higher standard than the rest of the party, let him have that instead)
He can grab himself Adapsin(.2 essence) and Biocompatibility (cyberwear), and then he can get cyberwear for effectively .5 Essence cost. 2 cyberlimbs, and 2 cyberlegs is 2 essence, Grab yourself some wired reflexes, (R3 would be 2.5 essence), R3 bone lacings is .75, and there you're sitting at .55 essence left and that is pretty damn cyber.
You could, alternatively, only have R2 Wired Reflexes (and some jazz to boost to that sweet sweet 5 initiative dice)(1.5 essence), and then you can grab yourself a Cyber Torso and Cyber Skull (.75 and .375 essence respectively), and that will total up to... Lets see... .2+2+1.5+.75+.75+.55... that equals out to 5.75 essence in total.
So, recount: 2 cyber arms, 2 cyber legs, rating 3 bone lacins, rating 2 Wired Reflexes, Cybre Skull, Cyber Torso, and Adapsin. That is a LOT of chrome, all with the only rule being bent is that you can buy Betawear at character creation.
The best part is, that shopping list I gave you totals up to 433,500. Prime Runner priority A Nuyen is 500,000, with the ability to get up to an additional 50K if they spend Karma on it (Even more if you go in debt).
that's 66500 to buy guns and stuff to fill your cyberlimbs which admittedly isn't a lot, but hey, you're chromed as fuck!