@ Anonymous,
I disagree to opinions of both above repliers.
Reasoning:
Had you both wanted 'families' to know of the intended marriage, Shillong would have been the place but it is not so and that is why you are here asking us for solution.
Solution:
1. Permanent address in India not having is not end of all hopes to get married. Special Marriage Act also says about couples permanent address but it is not binding. Had it been so, so many foreigners and/or OCI and or PIO card holders would not be getting married under Special Marriage Act in India by staying for minimum 30 days in India for the very purpose and get married and go happy about it later.
2. Both or either of you might be having some Residence Rent Deed of living in Mumbai from last 30 days?
3. Suggested to personally approach The Marriage Officer in the district, Mumbai where both of you presently reside on temporary basis and have a Rent deed and express the situation that both and or either of you are living at such and such residence address from last 30 days and here is rent deed of tenancy flat and wish to get married
4. In such case the Marriage Officer can seek Local Police verification report about the residence of one or both parties in his district, this process is being followed in various districts in other parts of India for couples who are more or less in your kind of situation yet want to marry under The Special Marriage Act.
5. You can also tell softly to The Marriage Registrar that its not crime living in rented premises if you want to marry under The Act and for verification about the actual living in the given address as mentioned in Rent Deed, he can always write official letter to Station House Officer of the local Police Station of the locality seeking "verification report" of your living there for more than one month on the date of your filling the notice for marriage (Statutory period as given in the SMA,1954 which is 30 days).
Hope above solution helps you both in getting married in Mumbai (one of its districts) under The Special Marriage Act!
[Seems to us case of intending couple from BPO (call center) industry]