If you are using Savoiardi biscuits then room temperature coffee is probably best for making sure the biscuits don't get too soggy. When I make tiramisu, I roll the biscuits quickly through a shallow saucer of strong coffee which doesn't stay hot for long.
I agree, however, that luke warm coffee just doesn't taste good. The volatile oils in coffee oxidise quickly and by the time it has cooled it will taste stale. Personally I substitute for a dark, non-creamy, coffee liqueur (Tia Maria works well).
I would also like to try using cold brewed coffee which, I'm told, has a different, sweeter flavour profile compared to normal brewed coffee.
Another experiment you could try is to cool the coffee as quickly as possible after brewing, say using a metal mixing bowl in an ice bath, to see if that helps reduce oxidisation. If you attempt this, then please report back.
If you are mixing coffee into the beaten egg white, then be sure to first mix it with the mascarpone which, I assume, you will fold into the egg whites. This way it won't deflate. I would use cold coffee for this, but warm or hot should be ok if it doesn't curdle the mascarpone.
Update: I've tried a variation on my suggestion to cool the coffee as quickly as possible by making, what is known as, Japanese iced coffee. This involves brewing the coffee directly over ice.
Normally a 1:1 ratio of liquid water to ice is used. I wanted it a bit stronger for tiramisu, so I periodically emptied out the brewed coffee so that it would be cold but not too dilute.
I'm very happy with the results. Even though I brewed the coffee a bit too far in advance, it was not stale 20 minutes later when I came to use it for the biscuits. The tiramisu was great too.