The lounge is average and tends to get crowded at night - it is a huge improvement from what it used to be - but 7 hours there is tough.
No free wifi and such - the food is quite good though - good chicken biryani on offer and comfy seats.

You can do one the following:
1) Mumbai airport has some spa's for foot reflexology and the like to kill an hour or so.

2) Go to one of the hotels between the two terminals, call them and see if they will give you a day use rate as many of them have international arrivals check-in at midnight or after. I would recommend calling the following:
a) Hyatt Regency Mumbai (2-5 mins from Intl airport, 15-25 from domestic depending on traffic)
b) The Leela Mumbai
c)ITC Grand Maratha
d) The Lalit Intercontinental
e) The Sahara Star (right at the domestic airport)

3) If none of these make sense you could go hang out at the Hyatt Regency and book a spa treatment which normally gives you access to the gym, wet areas (Sauna, steam, jacuzzi) - Or even just pay Rs.1000 for day access to the gym/pool/wet areas - its a great way to kill some time.

Thanks and have a good trip!