Maybe, but it wouldn't be nice like with Windows 2000 mount points (https://en.wikipedia.org/wiki/NTFS_volume_mount_point) as I'm pretty certain that 9x doesn't support the modern Windows kind of thing where you can go into Disk Management, remove a drive letter, then mount the drive elsewhere, unless there was an obscure tool you could get from elsewhere for that.
I think you could do it this way:
First, prevent Windows 9x from mounting the DOS partition by hiding it when Windows is running. Re: DOS 6.22 and Windows 98SE on one machine covers some options which other users on here have posted.
Then, in Windows 9x, use some kind of application to access the DOS partition.
It's possible that someone has made a Windows Explorer extension/addon which can make the partition show up in Explorer even though it doesn't have a drive letter, but I doubt it - it would be a lot of work and not something that many people would use.
I'm pretty sure that I got mtools (command line tool for accessing FAT filesystems) and https://mtoolsfm.sourceforge.net/ (GUI which uses mtools) running under some version of Windows XP so that I could graphically browse disk images. Perhaps they can be convinced to talk directly to a partition, and made to work on Windows 9x. perhaps they can be convinced to talk directly to a partition. I think I had to compile MToolsFM myself on Cygwin, and I compiled it using Win32 GTK libraries so I didn't need an X server to run it. It looks like I have that stuff in an old broken Cygwin install from 20 years ago, I can't seem to run it now.
Maybe there are nicer tools for accessing FAT filesystems from Windows 9x now, but there weren't back then because most people who were using Windows just had their FAT filesystems on their disks, which were mounted, and most people who had images of FAT filesystems were using Linux I guess!
This is all painful stuff though. Why do you want to not have it mounted, is it because you want Windows 9x to be/stay as C:?