There is no facility available in the SDK that will do what you want that I know of. However, you can simply find the edict_t of a particular client given his name, if you cycle through all of them and compare their netname with the supplied one.
Code:
int iClientIndex;
edict_t *pClient;
pClient = NULL; // initialize
// cycle through all clients...
for (iClientIndex = 1; iClientIndex < gpGlobals->maxClients; iClientIndex++)
{
pClient = INDEXENT (iClientIndex);
if (FNullEnt (pClient))
continue; // discard invalid client slots
if (strcmp ("two_masks", STRING (pClient->v.netname)) == 0)
break; // it's this client !
}
// have we finally found a client ?
if (!FNullEnt (pClient))
{
DoSomeStuffWithClient (pClient);
DoSomeOtherStuffWithClient (pClient);
SlapClientAroundABit (pClient);
MakeClientJumpUpAndDown (pClient);
ClientWriteALoveLetterToYourMomma (pClient);
// etc...
}