Oddity when inheriting Gmp.mpz

Test case:

string format_time(int t) {return
sprintf("%02d:%02d:%02d",t/3600,(t/60)%60,t%60);}

class Time1
{
    inherit Gmp.mpz;
    string _sprintf(int type,mapping|void params) {int
val=(int)mpz::this; write("-- %d\n",val); return type=='O' ?
format_time(val) : ::_sprintf(type,params);}
}

class Time2
{
    inherit Gmp.mpz;
    string _sprintf(int type,mapping|void params) {int
val=(int)mpz::this; write("-- %d\n",val); return type=='O' ?
(string)val : ::_sprintf(type,params);}
}

int main()
{
    write("Bad: %O\n",Time1(1234));
    write("Good: %O\n",Time2(1234));
}

I asked about this a while ago, and was advised to clean out heaps of
my code and replace it with simple inheritance from Gmp.mpz. This
works nicely in a simple case, but fails if I'm calling a function.

Am I going about this wrongly? All I need to do is retrieve the actual
integer stored in the current object, but simply (int)this isn't
working ("no cast method in object"). Using (int)mpz::this does give
correct results, but only as long as I don't pass it to another
function. (In the actual code, format_time() is a bit more
complicated, so simply inlining the sprintf into the time class isn't
ideal.)

It may be an over-eager optimization, but it's also likely that I'm
completely misunderstanding something here.

ChrisA