Thank you Mark, that's something I would call a 100% sure way. I've been reading though a number of sources yesterday and it looks like you are right.
Some vendors (like FEL www.felmicamps.co.uk) offer suitable devices (e.g., minijack-minijack mic preamp) but without a battery inside. Meaning that using them on my reader's line in is no use. The same FEL promise to launch a battery operated model soon, they call it an MP3 Mic Preamp, which would be suitable for devices without a mic input.
However, your comment about noise multiplication sounds a solid argument. Before giving up on ECM-MS907, I'll try to hunt for an affordable 2-channel mixer keeping that "noise issue" in mind.
Thanks again. I'll be posting further experiences.