I'd be interested in any replies you get to this.
Myself and my partner have no trouble repaying our current mortgage (and in fact overpay it) but we need to move if we want to have any kids. There is barely enough room for 2 adults, and no room at all for a child. Our figures for outstanding mortgage, and possible selling price, are very similar to yours.
I approached my financial institution (EBS) with the same plan as yourself - sell current home, pay the shortfall by way of a loan, and rent more suitable accomodation. They said they wouldn't allow a sale for less than the outstanding mortgage, and that I would need to give them a cheque for the shortfall to close the sale. They will not make arrangements for a personal loan.
I followed this up by way of an email to my branch manager, and also an email to their CEO. No joy, although the CEO was nicer in his reply by saying this was something they were looking into later this year - but for the meantime their focus was on people in neg equity who had repayment difficulties.
It is difficult to decide to realise an approx €70k loss in order to have children but we can't wait 10years for the economoy to recover. We probably have another 2-3years before we would have difficulties in having children so the decision is looming!!
Best of luck to you.