convert large integers to hex

Hi,

I'm wondering if someone has solved the problem of converting very
large integers to hex. I know about format.hexmode and as.hexmode, but
these rely on integers. The numbers I'm working with are overflowing
and losing precision. Here's an example:

x <- "6595137340052185552" # stored as character
as.integer(x) # warning about inaccurate conversion
format.hexmode(as.numeric(x)) # warnings about loss of precision
as.hexmode(x) # more warnings and does not do what I expected

I'm planning on writing a function that will do this, but would like
to know if anybody already has a solution. Basically, I would like the
functionality of format.hexmode on arbitrarily large integers.

Thanks,

--sundar