Dual boot Xubuntu and Slackware - load lilo while choosing Slackware

Xubuntu and Slack co-exist on one disk.
Please pay attention that both - Xubuntu and Slack are encrypted.

I use GRUB (installed under Xubuntu) to boot OSes - that works fine (both Xubuntu and Slack boot up).
Slackware has installed lilo on /dev/sda8 /boot as attempt to place it (by default) on /dev/cryptvg/root was giving following error:
Code:Fatal: device-mapper: only linear boot device supportedCode:
root@mini:~# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 35G 0 disk
|-sda1 8:1 0 50M 0 part
|-sda2 8:2 0 13.2G 0 part
|-sda3 8:3 0 499M 0 part
|-sda4 8:4 0 1K 0 part
|-sda5 8:5 0 476M 0 part /boot <-- Xubuntu
|-sda6 8:6 0 14G 0 part / <-- encrypted root - Xubuntu
|-sda7 8:7 0 100M 0 part
|-sda8 8:8 0 500M 0 part /boot <-- Slack
|-sda9 8:9 0 6G 0 part
| `-lukssda9 253:0 0 6G 0 crypt
| |-cryptvg-root 253:1 0 5.5G 0 lvm / <-- encrtpted root - Slack
| `-cryptvg-swap 253:2 0 508M 0 lvmThat works fine, however while choosing Slack in Grub, it boots system without stopping on lilo.
Question: how can I/can I let Grub load lilo while choosing "Slackware"