Congratulations on your commitment ceremony!!
Your plans sound like a combination of my wedding and my friend's wedding, I don't think there is anything at all wrong with wanting to do things your way, the way it suits you and the way your loved ones will know made you two happy on your special day.
My husband and I invited only our immediate families to our wedding, as we were so flat broke we had to hock a few bits and pieces from around the house just to afford our wedding rings!! We had a courthouse ceremony in front of our families and then went to dinner at a restaurant afterwards, it was hilarious explaining to the staff when we made the booking that, 'No, we don't want the function room, just a table for ten... yes, I did say 'wedding reception', it's going to be a very small one'.
And at my friend's wedding, she and her husband requested very politely on the invites that, because they had lived together for three years and had two kids together, there really wasn't much in the way of gifts that would be handy to them now, but if their friends and family could send them a money order/cheque with their RSVP for the $30 cost of the meal at the reception, that and their attendance on the day would be more than enough 'gift' for them. It went swimmingly, everyone enjoyed themselves (even me when my husband got hold of the open bar and ended up with his tie wrapped around his head Karate-Kid style), and a lovely time was had by all - and I think a lot of the not-so-well-off among us appreciated the fact that we only had to spend $30pp on dinner rather than trying to scrape together $100+ for a fancy gift.

There are some cute poems getting around on wedding invites these days about how the couple already has everything to make their house a home, so a small 'donation' to their 'honeymoon/house deposit/baby savings fund' would be more helpful to them, so maybe have a Google and see if you can find anything relevant to your situation??
Otherwise, just do what my friends did and say that your attendance and paying for your own meal/drinks is more than enough of a 'present' to you and your beloved. There's nothing impolite about wording it like that and people will probably be grateful that they are pretty much getting a cheaper deal than having to buy gifts (or, at least, they know they can choose something other than 'chicken or fish'!!).