You can mount ext3 as an 'ext2fs' but you may have to fsck_e2tfs it first. Resier you can mount
read only as an 'reasierfs'. You might want to check out the e2fsprogs and progsreiserfs ports in sysutils. I think FreeBSD has
experimental support for writing to reiserfs v3, but I'd avoid it unless you're ready for data loss (but hey, I'm paranoid).
I think modern Linux 2.6 should have READ ONLY support for UFS in it, but I dunno if it is enabled by default with any distros. Judging by a short google, I believe the command is:
mount -r -t ufs -o ufstype=ufs2 <device> <mount_dir>
but don't quote me.
No one can really help you with the boot loader issue unless you post details about your partitioning schema and BL setup. Some time ago I wrote a
thing on booting BSD from various boot loaders, with having Windows & Linux on the same disk being considered. Which also carefully notes something you will find in the FreeBSD FAQ.
You can boot Linux from from boot0, but you must have a boot loader like GRUB/LILO in the Linux / partitions boot sector in order for it to work. Other wise it has no way to find the Linux kernel, in the default config.