Ok, I found a time to try it, at last.
FPC 2.2.2:
The Windows version works without a notch.
The Linux version, hovever...
Linking ../../../chentrah
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_rateallocate':
tcd.c.text+0x1436): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_malloc_decode_tile':
tcd.c.text+0x21bb): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_init_encode':
tcd.c.text+0x2e76): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_malloc_encode':
tcd.c.text+0x392f): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_rateallocate':
tcd.c.text+0x1436): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_malloc_decode_tile':
tcd.c.text+0x21bb): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_init_encode':
tcd.c.text+0x2e76): undefined reference to `pow'
/e/cheb/chentrah/3rdparty/Imaging/Extras/Extensions/J2KObjects/libopenjpeglinx86.a(tcd.o): In function `tcd_malloc_encode':
tcd.c.text+0x392f): undefined reference to `pow'
/home/cheb/chentrah/modules/chentrah/src/chentrah.lpr(63,3) Error: Error while linking
My modified OpenJpeg with dynamic loading generates AV at first attempt to decode a file. I'll look into it further: looks like I missed something.

On the other hand, maybe I do it the wrong way. There is my build script:
BUILTIN=_test013

mkdir /tmp/.chentrah
rm /tmp/.chentrah/*

bin2obj -o cl_builtin_font.inc -c builtin_font font.png
bin2obj -o cl_console_font.inc -c iim_console consolefont.png
bin2obj -o cl_bgimg_error.inc -c error_bg_img cl_background_error.png
bin2obj -o cl_j2k_test.inc -c j2k_test_image cl_test.jp2

fpc extractdwrflnfo.pp -FE../../../bin/ -FU/tmp/.chentrah/
strip -s -x -X ../../../bin/extractdwrflnfo
fpc brutalstrip.pp -FE../../../bin/ -FU/tmp/.chentrah/
strip -s -x -X ../../../bin/brutalstrip
fpc incbuild.dpr -FE../../../bin/ -FU/tmp/.chentrah/
strip -s -x -X ../../../bin/incbuild

fpc chentrah.lpr -Xs -gw -XX -O3 -dcge -dnotlaz -OpPentium4 -OoUncertain -OoRegVar -OoPeepHole -Ooasmcse -Sd -Sh -Sc -Sg -Rintel -Fu./ -FE../../../ -FU/tmp/.chentrah/ -Fi../../${BUILTIN}/src/ -Fu../../${BUILTIN}/src/ -Fu../../../3rdparty/Imaging/Source/ -Fu../../../3rdparty/Imaging/Source/JpegLib/ -Fu../../../3rdparty/Imaging/Source/ZLib/ -Fi../../../3rdparty/Imaging/Source/ -Fu../../../3rdparty/Imaging/Extras/Extensions/ -Fu../../../3rdparty/Imaging/Extras/Extensions/J2KObjects/

../../../bin/incbuild build.h
../../../bin/extractdwrflnfo ../../../chentrah
mv ../../../chentrah.elf-zdli ../../../bin/
strip -s -x -X ../../../chentrah